diff --git a/SolutionPersistence.sln b/SolutionPersistence.sln
deleted file mode 100644
index 1c61458e..00000000
--- a/SolutionPersistence.sln
+++ /dev/null
@@ -1,68 +0,0 @@
-
-Microsoft Visual Studio Solution File, Format Version 12.00
-# Visual Studio Version 16
-VisualStudioVersion = 16.0.29322.22
-MinimumVisualStudioVersion = 15.0.26124.0
-Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution Items", "{1CE9670B-D5FF-46A7-9D00-24E70E6ED48B}"
-	ProjectSection(SolutionItems) = preProject
-		.editorconfig = .editorconfig
-		Directory.Build.props = Directory.Build.props
-		Directory.Build.targets = Directory.Build.targets
-		Directory.Packages.props = Directory.Packages.props
-		global.json = global.json
-		nuget.config = nuget.config
-		README.md = README.md
-		stylecop.json = stylecop.json
-		version.json = version.json
-	EndProjectSection
-EndProject
-Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{9E154A29-1796-4B85-BD81-B6A385D8FF71}"
-	ProjectSection(SolutionItems) = preProject
-		src\.editorconfig = src\.editorconfig
-		src\Directory.Build.props = src\Directory.Build.props
-		src\Directory.Build.targets = src\Directory.Build.targets
-	EndProjectSection
-EndProject
-Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "test", "test", "{36CCE840-6FE5-4DB9-A8D5-8CF3CB6D342A}"
-	ProjectSection(SolutionItems) = preProject
-		test\.editorconfig = test\.editorconfig
-		test\Directory.Build.props = test\Directory.Build.props
-		test\Directory.Build.targets = test\Directory.Build.targets
-	EndProjectSection
-EndProject
-Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{7743A30D-6296-44C3-B8FB-369D37566D94}"
-EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Microsoft.VisualStudio.SolutionPersistence", "src\Microsoft.VisualStudio.SolutionPersistence\Microsoft.VisualStudio.SolutionPersistence.csproj", "{43507C4F-E739-426F-8070-5260D668C3D9}"
-EndProject
-Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "test", "test", "{1EBF1959-C483-46AC-ADB6-5D5A72544EDF}"
-EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Microsoft.VisualStudio.SolutionPersistence.Tests", "test\Microsoft.VisualStudio.SolutionPersistence.Tests\Microsoft.VisualStudio.SolutionPersistence.Tests.csproj", "{9D224F03-C60B-49D5-84D9-FAF6913EFB9E}"
-EndProject
-Global
-	GlobalSection(SolutionConfigurationPlatforms) = preSolution
-		Debug|Any CPU = Debug|Any CPU
-		Release|Any CPU = Release|Any CPU
-	EndGlobalSection
-	GlobalSection(ProjectConfigurationPlatforms) = postSolution
-		{43507C4F-E739-426F-8070-5260D668C3D9}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
-		{43507C4F-E739-426F-8070-5260D668C3D9}.Debug|Any CPU.Build.0 = Debug|Any CPU
-		{43507C4F-E739-426F-8070-5260D668C3D9}.Release|Any CPU.ActiveCfg = Release|Any CPU
-		{43507C4F-E739-426F-8070-5260D668C3D9}.Release|Any CPU.Build.0 = Release|Any CPU
-		{9D224F03-C60B-49D5-84D9-FAF6913EFB9E}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
-		{9D224F03-C60B-49D5-84D9-FAF6913EFB9E}.Debug|Any CPU.Build.0 = Debug|Any CPU
-		{9D224F03-C60B-49D5-84D9-FAF6913EFB9E}.Release|Any CPU.ActiveCfg = Release|Any CPU
-		{9D224F03-C60B-49D5-84D9-FAF6913EFB9E}.Release|Any CPU.Build.0 = Release|Any CPU
-	EndGlobalSection
-	GlobalSection(SolutionProperties) = preSolution
-		HideSolutionNode = FALSE
-	EndGlobalSection
-	GlobalSection(NestedProjects) = preSolution
-		{9E154A29-1796-4B85-BD81-B6A385D8FF71} = {1CE9670B-D5FF-46A7-9D00-24E70E6ED48B}
-		{36CCE840-6FE5-4DB9-A8D5-8CF3CB6D342A} = {1CE9670B-D5FF-46A7-9D00-24E70E6ED48B}
-		{43507C4F-E739-426F-8070-5260D668C3D9} = {7743A30D-6296-44C3-B8FB-369D37566D94}
-		{9D224F03-C60B-49D5-84D9-FAF6913EFB9E} = {1EBF1959-C483-46AC-ADB6-5D5A72544EDF}
-	EndGlobalSection
-	GlobalSection(ExtensibilityGlobals) = postSolution
-		SolutionGuid = {E3944F6A-384B-4B0F-B93F-3BD513DC57BD}
-	EndGlobalSection
-EndGlobal
diff --git a/global.json b/global.json
index 20b589f8..6306f36b 100644
--- a/global.json
+++ b/global.json
@@ -1,6 +1,6 @@
 {
   "sdk": {
-    "version": "9.0.102",
+    "version": "9.0.200",
     "rollForward": "patch",
     "allowPrerelease": false
   },
diff --git a/src/Microsoft.VisualStudio.SolutionPersistence/Serializer/SlnV12/SlnFileV12Serializer.Reader.cs b/src/Microsoft.VisualStudio.SolutionPersistence/Serializer/SlnV12/SlnFileV12Serializer.Reader.cs
index ed558a64..be070214 100644
--- a/src/Microsoft.VisualStudio.SolutionPersistence/Serializer/SlnV12/SlnFileV12Serializer.Reader.cs
+++ b/src/Microsoft.VisualStudio.SolutionPersistence/Serializer/SlnV12/SlnFileV12Serializer.Reader.cs
@@ -69,6 +69,11 @@ internal ValueTask<SolutionModel> ParseAsync(ISolutionSerializer serializer, str
             // Some property bags need to be loaded after all projects have been resolved.
             List<(SolutionItemModel, SolutionPropertyBag)> delayLoadProperties = [];
 
+            // Keep track of projects with duplicate ids so that we can
+            // duplicate the configuration after all projects have been loaded.
+            // This preserves the orginial buggy behavior of the solution parser.
+            List<(Guid NewId, SolutionProjectModel DuplicateProject)> fixedProjectIds = [];
+
             try
             {
                 using (solutionModel.SuspendProjectValidation())
@@ -87,7 +92,7 @@ internal ValueTask<SolutionModel> ParseAsync(ISolutionSerializer serializer, str
                             case LineType.Project:
                                 this.TarnishIf(inProject);
                                 inProject = true;
-                                currentProject = this.ReadProjectInfo(solutionModel, ref tokenizer);
+                                currentProject = this.ReadProjectInfo(solutionModel, ref tokenizer, fixedProjectIds);
                                 break;
 
                             case LineType.EndProject:
@@ -200,6 +205,17 @@ internal ValueTask<SolutionModel> ParseAsync(ISolutionSerializer serializer, str
                         this.TarnishIf(!item.AddSlnProperties(properties));
                     }
 
+                    foreach ((Guid newId, SolutionProjectModel duplicateProject) in fixedProjectIds)
+                    {
+                        // The newly generated id will not have configurations, so make a
+                        // copy of the original project configurations.
+                        // This preserves the orginial buggy behavior of the solution parser.
+                        if (solutionModel.FindItemById(newId) is SolutionProjectModel project)
+                        {
+                            project.ProjectConfigurationRules = duplicateProject.ProjectConfigurationRules;
+                        }
+                    }
+
                     VisualStudioProperties vsProperties = solutionModel.VisualStudioProperties;
                     vsProperties.OpenWith = CommentToOpenWithVS(openWithVsVersion.AsSpan());
                     vsProperties.MinimumVersion = minVsVersion;
@@ -218,6 +234,9 @@ internal ValueTask<SolutionModel> ParseAsync(ISolutionSerializer serializer, str
 
             return new ValueTask<SolutionModel>(solutionModel);
 
+            // Adds the property bag to the project or folder.
+            // Handles special cases for the sln parser.
+            // Returns false if there was an error reading the properties and the solution file should be considered tarnished.
             static bool AddProjectProperties(
                 SolutionItemModel? currentProject,
                 SolutionPropertyBag? currentPropertyBag,
@@ -498,7 +517,7 @@ private bool ReadLine(out StringTokenizer lineScanner)
             return checkOnly ? null : new SolutionPropertyBag(sectionName.ToString(), scope);
         }
 
-        private SolutionItemModel ReadProjectInfo(SolutionModel solution, ref StringTokenizer tokenizer)
+        private SolutionItemModel ReadProjectInfo(SolutionModel solution, ref StringTokenizer tokenizer, List<(Guid NewId, SolutionProjectModel DuplicateProject)> fixedProjectIds)
         {
             // Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "App1", "App1\App1.csproj", "{B0D4AB54-EB86-4C88-A2A4-C55D0C200244}"
             //         ^  <- this is tokenizer pos.
@@ -527,6 +546,8 @@ private SolutionItemModel ReadProjectInfo(SolutionModel solution, ref StringToke
             this.SolutionAssert(!projectUniqueId.IsEmpty, Errors.MissingProjectId);
             this.TarnishIf(!Guid.TryParse(projectUniqueId.ToString(), out Guid projectId));
 
+            SolutionItemModel? duplicateItem = solution.FindItemById(projectId);
+
             if (projectTypeId == ProjectTypeTable.SolutionFolder)
             {
 #pragma warning disable CS0618 // Type or member is obsolete (Temporaily create a potentially invalid solution folder until nested projects is interpreted.)
@@ -534,7 +555,7 @@ private SolutionItemModel ReadProjectInfo(SolutionModel solution, ref StringToke
 #pragma warning restore CS0618 // Type or member is obsolete
 
                 // Solution folders with duplicate ids should not error when reading sln files to preserve legacy behavior.
-                if (solution.FindItemById(projectId) is not null)
+                if (duplicateItem is not null)
                 {
                     projectId = Guid.NewGuid();
                     this.tarnished = true;
@@ -545,9 +566,27 @@ private SolutionItemModel ReadProjectInfo(SolutionModel solution, ref StringToke
             }
             else
             {
+                string path = PathExtensions.ConvertBackslashToModel(relativePath.ToString());
+
+                // This should error, or remove any configuration associated with the duplicate id.
+                // However some old parsers would just ignore the duplicate id and continue.
+                if (duplicateItem is SolutionFolderModel duplicateFolder)
+                {
+                    duplicateFolder.Id = Guid.NewGuid();
+                    this.tarnished = true;
+                }
+                else if (duplicateItem is SolutionProjectModel duplicateProject)
+                {
+                    projectId = CreateNewProjectId(solution, path);
+                    this.tarnished = true;
+
+                    // Record the new project id so it's configuration can be duplicated.
+                    fixedProjectIds.Add((projectId, duplicateProject));
+                }
+
 #pragma warning disable CS0618 // Type or member is obsolete (Temporaily create a potentially invalid solution folder until nested projects is interpreted.)
                 SolutionProjectModel project = solution.AddSlnProject(
-                    filePath: PathExtensions.ConvertBackslashToModel(relativePath.ToString()),
+                    filePath: path,
                     projectTypeId: projectTypeId,
                     folder: null);
 #pragma warning restore CS0618 // Type or member is obsolete
@@ -555,6 +594,13 @@ private SolutionItemModel ReadProjectInfo(SolutionModel solution, ref StringToke
                 project.DisplayName = displayName.ToString();
                 return project;
             }
+
+            // If creating a new project id, go ahead and try to use the slnx default id.
+            static Guid CreateNewProjectId(SolutionModel solution, string path)
+            {
+                Guid id = DefaultIdGenerator.CreateIdFrom(path);
+                return solution.FindItemById(id) is null ? id : Guid.NewGuid();
+            }
         }
 
         // Condition that would mark solution file as "tarnished"
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/Format.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/Format.cs
index 93cbf323..911d3ff2 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/Format.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/Format.cs
@@ -14,12 +14,6 @@ public class Format
     [Fact]
     public async Task ConvertASCIItoUTF8Async()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         const string asciiFolderName = "directory123";
         const string utf8FolderName = "répertoire123";
 
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/IdGenerator.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/IdGenerator.cs
index a382ba22..ccf0133a 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/IdGenerator.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/IdGenerator.cs
@@ -77,12 +77,6 @@ public void CheckGeneratedIds()
     [Fact]
     public async Task CheckModelIds()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         SolutionModel solution = await SolutionSerializers.SlnXml.OpenAsync(SlnAssets.XmlSlnxMany.Stream, CancellationToken.None);
 
         SolutionProjectModel? urlProject = solution.FindProject("http://localhost:8080");
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/InvalidSolutions.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/InvalidSolutions.cs
index b5fc7a13..6a36ff37 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/InvalidSolutions.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/InvalidSolutions.cs
@@ -29,12 +29,6 @@ public Task InvalidXmlSlnxAsync()
     [Fact]
     public async Task InvalidSlnxAsync()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         ResourceStream wrongRoot = SlnAssets.LoadResource("Invalid/WrongRoot.slnx");
         string wrongRootFile = wrongRoot.SaveResourceToTempFile();
 
@@ -55,12 +49,6 @@ public async Task InvalidSlnxAsync()
     [Fact]
     public async Task InvalidSlnAsync()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         ResourceStream invalidSln = SlnAssets.LoadResource("Invalid/Invalid.sln");
         string invalidSlnFile = invalidSln.SaveResourceToTempFile();
 
@@ -239,4 +227,96 @@ public async Task DuplicateFolderIdSlnxAsync()
         Assert.Equal(3, ex.Line);
         Assert.Equal(4, ex.Column);
     }
+
+    /// <summary>
+    /// Ensure the slnx parser does fail if project id collides with folder id.
+    /// Expected behavior is the folder id is changed to a new value.
+    /// </summary>
+    [Fact]
+    public async Task DuplicateItemIdSlnAsync()
+    {
+        ResourceStream duplicateId = SlnAssets.LoadResource("Invalid/DuplicateItemId.sln");
+        SolutionModel solution = await SolutionSerializers.SlnFileV12.OpenAsync(duplicateId.Stream, CancellationToken.None);
+        Assert.NotNull(solution.SerializerExtension);
+        Assert.True(solution.SerializerExtension.Tarnished);
+
+        SolutionProjectModel? projectModel = solution.FindProject("DuplicateProjectId.csproj");
+        Assert.NotNull(projectModel);
+        Assert.Equal(projectModel.Id, new Guid("11111111-1111-1111-1111-111111111111"));
+
+        SolutionFolderModel? folderModel = solution.FindFolder("/DuplicateFolderId/");
+        Assert.NotNull(folderModel);
+        Assert.NotEqual(folderModel.Id, new Guid("11111111-1111-1111-1111-111111111111"));
+    }
+
+    /// <summary>
+    /// Recreate legacy bug behavior when duplicate project id guids are found in .sln files.
+    /// Use corrupt configuration and change second project id to new value.
+    /// </summary>
+    [Fact]
+    public async Task DuplicateProjectIdSlnAsync()
+    {
+        ResourceStream duplicateId = SlnAssets.LoadResource("Invalid/DuplicateProjectId.sln");
+        SolutionModel solution = await SolutionSerializers.SlnFileV12.OpenAsync(duplicateId.Stream, CancellationToken.None);
+        Assert.NotNull(solution.SerializerExtension);
+        Assert.True(solution.SerializerExtension.Tarnished);
+
+        SolutionProjectModel? project = solution.FindProject("DuplicateProjectId.csproj");
+        Assert.NotNull(project);
+        Assert.Equal(project.Id, new Guid("8BADBEEF-1111-2222-3333-444444444444"));
+
+        SolutionProjectModel? project2 = solution.FindProject("DuplicateProjectIdOpposite.csproj");
+        Assert.NotNull(project2);
+        Assert.NotEqual(project2.Id, new Guid("8BADBEEF-2222-3333-4444-555555555555"));
+
+        (string? buildType, string? platform, bool build, bool deploy) = project.GetProjectConfiguration(BuildTypeNames.Debug, PlatformNames.AnySpaceCPU);
+
+        (string? buildType2, string? platform2, bool build2, bool deploy2) = project2.GetProjectConfiguration(BuildTypeNames.Debug, PlatformNames.AnySpaceCPU);
+
+        // The configuration from the "opposite" project should be used by both as it appears last in the file.
+        Assert.Equal(BuildTypeNames.Release, buildType);
+        Assert.Equal(PlatformNames.AnySpaceCPU, platform);
+        Assert.True(build);
+        Assert.False(deploy);
+
+        // The configuration from the "opposite" project should be used by both as it appears last in the file.
+        Assert.Equal(BuildTypeNames.Release, buildType2);
+        Assert.Equal(PlatformNames.AnySpaceCPU, platform2);
+        Assert.True(build2);
+        Assert.False(deploy2);
+
+        // Compare the new model to the 'After' sln (Change the new Guid to FFFF to match).
+        project2.Id = new Guid("FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF");
+        ResourceStream duplicateIdAfter = SlnAssets.LoadResource("DuplicateProjectId-After.sln");
+        AssertSolutionsAreEqual(duplicateIdAfter.ToLines(), await solution.ToLinesAsync(SolutionSerializers.SlnFileV12));
+    }
+
+    /// <summary>
+    /// Ensure the slnx parser does fail on duplicate project guids.
+    /// </summary>
+    [Fact]
+    public async Task DuplicateProjectIdSlnxAsync()
+    {
+        ResourceStream duplicateId = SlnAssets.LoadResource("Invalid/DuplicateProjectId.slnx");
+        SolutionException ex = await Assert.ThrowsAsync<SolutionException>(
+            async () => _ = await SolutionSerializers.SlnXml.OpenAsync(duplicateId.Stream, CancellationToken.None));
+
+        Assert.StartsWith(string.Format(Errors.DuplicateItemRef_Args2, "8badbeef-1111-2222-3333-444444444444", nameof(SolutionProjectModel)), ex.Message);
+        Assert.Equal(5, ex.Line);
+        Assert.Equal(6, ex.Column);
+    }
+
+    /// <summary>
+    /// Checks that an error is thrown if the solution contains an invalid project type id.
+    /// </summary>
+    [Fact]
+    public async Task InvalidProjectTypeSlnAsync()
+    {
+        ResourceStream invalidProjectType = SlnAssets.LoadResource("Invalid/InvalidProjectType.sln");
+        SolutionException ex = await Assert.ThrowsAsync<SolutionException>(
+            async () => _ = await SolutionSerializers.SlnFileV12.OpenAsync(invalidProjectType.Stream, CancellationToken.None));
+        Assert.StartsWith(Errors.InvalidProjectType, ex.Message);
+        Assert.Equal(5, ex.Line);
+        Assert.Null(ex.Column);
+    }
 }
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/LegacyValues.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/LegacyValues.cs
index d1298411..87624b4b 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/LegacyValues.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/LegacyValues.cs
@@ -40,12 +40,6 @@ public async Task TrimVisualStudioPropertiesAsync()
     [Fact]
     public async Task IgnoreDisplayNameSlnx()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         await ValidateModifiedSolutionAsync(CreateModifiedModel, SlnAssets.XmlSlnxSingleNativeProject, SlnAssets.XmlSlnxSingleNativeProject);
 
         static void CreateModifiedModel(SolutionModel solution)
@@ -64,12 +58,6 @@ static void CreateModifiedModel(SolutionModel solution)
     [Fact]
     public async Task IgnoreDisplayNameSln()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         await ValidateModifiedSolutionAsync(CreateModifiedModel, SlnAssets.ClassicSlnSingleNativeProject, SlnAssets.ClassicSlnSingleNativeProject, SolutionSerializers.SlnFileV12);
 
         static void CreateModifiedModel(SolutionModel solution)
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/MakeSlnx.MakeSlnxFixture.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/MakeSlnx.MakeSlnxFixture.cs
index e145774e..80c6ce15 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/MakeSlnx.MakeSlnxFixture.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/MakeSlnx.MakeSlnxFixture.cs
@@ -24,7 +24,10 @@ public class MakeSlnxFixture
         /// </summary>
         public MakeSlnxFixture()
         {
-            string outputDirectory = Path.Combine(Path.GetTempPath(), OutputDirectory);
+            // Split output based on the .NET version (so we can run tests in parallel)
+            string netVersion = "net" + Environment.Version.ToString(3);
+
+            string outputDirectory = Path.Combine(Path.GetTempPath(), OutputDirectory, netVersion);
             if (Directory.Exists(outputDirectory))
             {
                 Directory.Delete(outputDirectory, true);
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/MakeSlnx.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/MakeSlnx.cs
index e3a3cf77..efa18630 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/MakeSlnx.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/MakeSlnx.cs
@@ -32,12 +32,6 @@ public sealed partial class MakeSlnx(MakeSlnx.MakeSlnxFixture fixture) : IClassF
     [Trait("TestCategory", "FailsInCloudTest")]
     public async Task ConvertSlnToSlnxAsync(ResourceName slnFileName)
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         ResourceStream slnFile = slnFileName.Load();
         int sampleFileSize = checked((int)slnFile.Stream.Length);
         string slnToSlnxFile = Path.ChangeExtension(Path.Join(fixture.SlnToSlnxDirectory, slnFile.Name), SolutionSerializers.SlnXml.DefaultFileExtension);
@@ -65,12 +59,6 @@ public async Task ConvertSlnToSlnxAsync(ResourceName slnFileName)
     [Trait("TestCategory", "FailsInCloudTest")]
     public async Task ConvertSlnxToSlnAsync(ResourceName slnxFileName)
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         ResourceStream slnxFile = slnxFileName.Load();
 
         string slnxToSlnFile = Path.ChangeExtension(Path.Join(fixture.SlnxToSlnDirectory, slnxFile.Name), SolutionSerializers.SlnFileV12.DefaultFileExtension);
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/ManipulateXmlKitchenSink.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/ManipulateXmlKitchenSink.cs
index b3fdd143..7d63791f 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/ManipulateXmlKitchenSink.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/ManipulateXmlKitchenSink.cs
@@ -20,12 +20,6 @@ public class ManipulateXmlKitchenSink
     [Fact]
     public async Task AddConfigurations()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         await ValidateModifiedSolutionAsync(
             CreateModifiedModel,
             SlnAssets.LoadResource("SlnxWhitespace/KitchenSink.slnx"),
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/PathSlashes.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/PathSlashes.cs
index 2bd2c228..3a604976 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/PathSlashes.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/PathSlashes.cs
@@ -17,12 +17,6 @@ public sealed class PathSlashes
     [Fact]
     public async Task VerifyModelSlahesAsync()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         ResourceStream slashesResource = SlnAssets.LoadResource("Invalid/PathSlashes.slnx");
 
         SolutionModel solution = await SolutionSerializers.SlnXml.OpenAsync(slashesResource.Stream, CancellationToken.None);
@@ -64,12 +58,6 @@ public async Task VerifyModelSlahesAsync()
     [Fact]
     public async Task ProjectReferencesFixedUpAsync()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         FileContents fixedSlashesResource = SlnAssets.LoadResource("PathSlashes.slnx").ToLines();
 
         // Save the Model back to stream.
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/Project.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/Project.cs
index d63dc4ee..d8b624b2 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/Project.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/Project.cs
@@ -58,12 +58,6 @@ public void AddProject()
     [Fact]
     public async Task RemoveProjectAsync()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         SolutionModel solution = await SolutionSerializers.SlnXml.OpenAsync(SlnAssets.XmlSlnxEverything.Stream, CancellationToken.None);
 
         string toRemove = Path.Join("src", "CoreMauiApp", "CoreMauiApp.csproj");
@@ -84,12 +78,6 @@ public async Task RemoveProjectAsync()
     [Fact]
     public async Task MoveProjectAsync()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         SolutionModel solution = await SolutionSerializers.SlnXml.OpenAsync(SlnAssets.XmlSlnxEverything.Stream, CancellationToken.None);
 
         string toMove = Path.Join("BlazorApp1", "BlazorApp1.csproj");
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripClassicSln.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripClassicSln.cs
index 5e6c80f4..5c495e30 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripClassicSln.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripClassicSln.cs
@@ -56,12 +56,6 @@ public Task AllClassicSolutionAsync(ResourceName sampleFile)
     /// <returns>Task to track the asynchronous call status.</returns>
     private static async Task TestRoundTripSerializerAsync(ResourceStream slnStream)
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         FileContents originalSolution = slnStream.ToLines();
 
         // Open the Model from stream.
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripClassicSlnThruSlnxStream.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripClassicSlnThruSlnxStream.cs
index dc44b54f..4580cd11 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripClassicSlnThruSlnxStream.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripClassicSlnThruSlnxStream.cs
@@ -46,12 +46,6 @@ private static async Task TestRoundTripSerializerAsync(
         ResourceStream slnStream,
         ResourceStream viaSlnxStream)
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         FileContents originalSolution = slnStream.ToLines();
 
         // Open the Model from stream.
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripXmlSlnx.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripXmlSlnx.cs
index 2b86b671..6a588175 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripXmlSlnx.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripXmlSlnx.cs
@@ -60,12 +60,6 @@ public Task AllXmlSolutionAsync(ResourceName sampleFile)
 
     private static async Task TestRoundTripSerializerAsync(ResourceStream slnStream)
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         // Open the Model from stream.
         SolutionModel model = await SolutionSerializers.SlnXml.OpenAsync(slnStream.Stream, CancellationToken.None);
         AssertNotTarnished(model);
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripXmlSlnxThruModelCopy.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripXmlSlnxThruModelCopy.cs
index 24987ff0..1f79e3e3 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripXmlSlnxThruModelCopy.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/RoundTripXmlSlnxThruModelCopy.cs
@@ -16,12 +16,6 @@ public class RoundTripXmlSlnxThruModelCopy
     [Fact]
     public Task CommentsAsync()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return Task.CompletedTask;
-        }
-
         return Assert.ThrowsAsync<FailException>(() => TestRoundTripSerializerAsync(SlnAssets.XmlSlnxComments));
     }
 
@@ -57,12 +51,6 @@ public Task CommentsAsync()
 
     private static async Task TestRoundTripSerializerAsync(ResourceStream slnStream)
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         // Open the Model from stream.
         SolutionModel model = await SolutionSerializers.SlnXml.OpenAsync(slnStream.Stream, CancellationToken.None);
         AssertNotTarnished(model);
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/Samples.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/Samples.cs
index 061ff31b..a8d08974 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/Samples.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Serialization/Samples.cs
@@ -14,12 +14,6 @@ public class Samples
     [Fact]
     public async Task ConvertToSlnx()
     {
-        if (IsMono)
-        {
-            // Mono is not supported.
-            return;
-        }
-
         string filePath = SlnAssets.ClassicSlnEverything.SaveResourceToTempFile();
         string slnxFilePath = Path.ChangeExtension(filePath, SolutionSerializers.SlnXml.DefaultFileExtension);
 
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/DuplicateProjectId-After.sln.txt b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/DuplicateProjectId-After.sln.txt
new file mode 100644
index 00000000..c8584a59
--- /dev/null
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/DuplicateProjectId-After.sln.txt
@@ -0,0 +1,30 @@
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.14.35819.311
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "DuplicateProjectId", "DuplicateProjectId.csproj", "{8BADBEEF-1111-2222-3333-444444444444}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "DuplicateProjectIdOpposite", "DuplicateProjectIdOpposite.csproj", "{FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{8BADBEEF-1111-2222-3333-444444444444}.Debug|Any CPU.ActiveCfg = Release|Any CPU
+		{8BADBEEF-1111-2222-3333-444444444444}.Debug|Any CPU.Build.0 = Release|Any CPU
+		{8BADBEEF-1111-2222-3333-444444444444}.Release|Any CPU.ActiveCfg = Debug|Any CPU
+		{8BADBEEF-1111-2222-3333-444444444444}.Release|Any CPU.Build.0 = Debug|Any CPU
+		{FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF}.Debug|Any CPU.ActiveCfg = Release|Any CPU
+		{FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF}.Debug|Any CPU.Build.0 = Release|Any CPU
+		{FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF}.Release|Any CPU.ActiveCfg = Debug|Any CPU
+		{FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF}.Release|Any CPU.Build.0 = Debug|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {E0F1003A-31EB-42B8-8684-F82D4D7B9108}
+	EndGlobalSection
+EndGlobal
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/DuplicateItemId.sln.txt b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/DuplicateItemId.sln.txt
new file mode 100644
index 00000000..a7af068b
--- /dev/null
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/DuplicateItemId.sln.txt
@@ -0,0 +1,26 @@
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.14.35819.311
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "DuplicateFolderId", "DuplicateFolderId", "{11111111-1111-1111-1111-111111111111}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "DuplicateProjectId", "DuplicateProjectId.csproj", "{11111111-1111-1111-1111-111111111111}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{11111111-1111-1111-1111-111111111111}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{11111111-1111-1111-1111-111111111111}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{11111111-1111-1111-1111-111111111111}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{11111111-1111-1111-1111-111111111111}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {E0F1003A-31EB-42B8-8684-F82D4D7B9108}
+	EndGlobalSection
+EndGlobal
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/DuplicateProjectId.sln.txt b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/DuplicateProjectId.sln.txt
new file mode 100644
index 00000000..0ce6ccc5
--- /dev/null
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/DuplicateProjectId.sln.txt
@@ -0,0 +1,30 @@
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.14.35819.311
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "DuplicateProjectId", "DuplicateProjectId.csproj", "{8BADBEEF-1111-2222-3333-444444444444}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "DuplicateProjectIdOpposite", "DuplicateProjectIdOpposite.csproj", "{8BADBEEF-1111-2222-3333-444444444444}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{8BADBEEF-1111-2222-3333-444444444444}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{8BADBEEF-1111-2222-3333-444444444444}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{8BADBEEF-1111-2222-3333-444444444444}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{8BADBEEF-1111-2222-3333-444444444444}.Release|Any CPU.Build.0 = Release|Any CPU
+		{8BADBEEF-1111-2222-3333-444444444444}.Debug|Any CPU.ActiveCfg = Release|Any CPU
+		{8BADBEEF-1111-2222-3333-444444444444}.Debug|Any CPU.Build.0 = Release|Any CPU
+		{8BADBEEF-1111-2222-3333-444444444444}.Release|Any CPU.ActiveCfg = Debug|Any CPU
+		{8BADBEEF-1111-2222-3333-444444444444}.Release|Any CPU.Build.0 = Debug|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {E0F1003A-31EB-42B8-8684-F82D4D7B9108}
+	EndGlobalSection
+EndGlobal
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/DuplicateProjectId.slnx.xml b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/DuplicateProjectId.slnx.xml
new file mode 100644
index 00000000..5f6baea4
--- /dev/null
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/DuplicateProjectId.slnx.xml
@@ -0,0 +1,9 @@
+<Solution>
+    <Project Path="DuplicateProjectId.csproj" Id="8badbeef-1111-2222-3333-444444444444" />
+
+
+    <Project Path="DuplicateProjectIdOpposite.csproj" Id="8badbeef-1111-2222-3333-444444444444">
+        <BuildType Solution="Debug|*" Project="Release" />
+        <BuildType Solution="Release|*" Project="Debug" />
+    </Project>
+</Solution>
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/InvalidProjectType.sln.txt b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/InvalidProjectType.sln.txt
new file mode 100644
index 00000000..0d26d91d
--- /dev/null
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/SlnAssets/Invalid/InvalidProjectType.sln.txt
@@ -0,0 +1,18 @@
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.14.35819.311
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("NotAGuid") = "InvalidProjectType", "InvalidProjectType.csproj", "{8A1FD751-4BAA-467E-A9FA-77239CA44A61}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {CFB61351-9264-45B3-BBE0-BAC5D49C87CE}
+	EndGlobalSection
+EndGlobal
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Utilities/SlnAssets.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Utilities/SlnAssets.cs
index 9aef8f25..093f5ad5 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Utilities/SlnAssets.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Utilities/SlnAssets.cs
@@ -13,6 +13,8 @@ internal static class SlnAssets
 
     public static ResourceName[] ClassicSlnFiles => GetAllSampleFiles(".sln").ToArray();
 
+    public static ResourceName[] XmlSlnxFiles => GetAllSampleFiles(".slnx").ToArray();
+
     #region Sample Classic Sln
 
     // Empty VS Solution template. ASCII encoding.
@@ -97,8 +99,6 @@ internal static class SlnAssets
     // A solution with missing configurations.
     public static ResourceStream XmlSlnxMissingConfigurations => LoadResource("Configurations/MissingConfigurations.slnx");
 
-    public static ResourceName[] XmlSlnxFiles => GetAllSampleFiles(".slnx").ToArray();
-
     #endregion
 
     #region Test result Slnx
diff --git a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Utilities/SlnTestHelper.cs b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Utilities/SlnTestHelper.cs
index ec1d182e..5497e8ae 100644
--- a/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Utilities/SlnTestHelper.cs
+++ b/test/Microsoft.VisualStudio.SolutionPersistence.Tests/Utilities/SlnTestHelper.cs
@@ -11,15 +11,6 @@ namespace Utilities;
 /// </summary>
 internal static class SlnTestHelper
 {
-    /// <summary>
-    /// We do not support mono, but xunit testing tries to run the .NET Framework tests on mono when running on Linux/Mac.
-    /// </summary>
-#if NETFRAMEWORK
-    internal static bool IsMono = Environment.OSVersion.Platform != PlatformID.Win32NT;
-#else
-    internal static bool IsMono = false;
-#endif
-
     /// <summary>
     /// Converts the model to it's file contents using the specified serializer.
     /// </summary>