diff --git a/lib/jsonapi/renderer/resources_processor.rb b/lib/jsonapi/renderer/resources_processor.rb
index dfd5264..e164018 100644
--- a/lib/jsonapi/renderer/resources_processor.rb
+++ b/lib/jsonapi/renderer/resources_processor.rb
@@ -50,12 +50,42 @@ def traverse_resource(res, include_keys, primary)
 
         if @include_rels.include?(ri)
           @include_rels[ri].merge!(keys_hash)
+
+          include_duplicate_resource(res) unless primary
         else
           @include_rels[ri] = keys_hash
           (primary ? @primary : @included) << res
         end
       end
 
+      def include_duplicate_resource(res)
+        duplicate_index = find_included_duplicate_resource(res)
+
+        return unless duplicate_index
+
+        duplicate = @included.delete_at(duplicate_index)
+
+        if duplicate.is_a?(Array)
+          duplicate << res
+        else
+          duplicate = [duplicate, res]
+        end
+
+        @included << duplicate
+      end
+
+      def find_included_duplicate_resource(res)
+        @included.find_index do |included|
+          if included.is_a?(Array)
+            unless included.empty?
+              included.first.jsonapi_type == res.jsonapi_type && included.first.jsonapi_id == res.jsonapi_id
+            end
+          else
+            included.jsonapi_type == res.jsonapi_type && included.jsonapi_id == res.jsonapi_id
+          end
+        end
+      end
+
       def enqueue_related_resources(res, prefix, include_dir)
         res.jsonapi_related(include_dir.keys).each do |key, data|
           child_prefix = "#{prefix}.#{key}".freeze
diff --git a/lib/jsonapi/renderer/simple_resources_processor.rb b/lib/jsonapi/renderer/simple_resources_processor.rb
index 1249e6d..13b3b27 100644
--- a/lib/jsonapi/renderer/simple_resources_processor.rb
+++ b/lib/jsonapi/renderer/simple_resources_processor.rb
@@ -7,13 +7,52 @@ class SimpleResourcesProcessor < ResourcesProcessor
       def process_resources
         [@primary, @included].each do |resources|
           resources.map! do |res|
-            ri = [res.jsonapi_type, res.jsonapi_id]
-            include_dir = @include_rels[ri].keys
-            fields = @fields[res.jsonapi_type.to_sym]
-            res.as_jsonapi(include: include_dir, fields: fields)
+            # Duplicates array
+            if res.is_a?(Array)
+              process_duplicates(res)
+            # Regular resource case
+            else
+              process_resource(res)
+            end
           end
         end
       end
+
+      def process_resource(resource)
+        ri = [resource.jsonapi_type, resource.jsonapi_id]
+        include_dir = @include_rels[ri].keys
+        fields = @fields[resource.jsonapi_type.to_sym]
+        resource.as_jsonapi(include: include_dir, fields: fields)
+      end
+
+      def process_duplicates(duplicates)
+        return unless duplicates.is_a?(Array) && duplicates.any?
+
+        duplicates.inject({}) do |result, duplicate|
+          if result.empty?
+            result = process_resource(duplicate) if result.empty?
+          else
+            duplicate_result = process_resource(duplicate)
+            result = deep_merge_duplicate_hashes(result, duplicate_result)
+          end
+        end
+      end
+
+      def deep_merge_duplicate_hashes(hash1, hash2)
+        merger = proc do |_, v1, v2|
+          if v1.is_a?(Hash) && v2.is_a?(Hash)
+            v1.merge(v2, &merger)
+          elsif v1.is_a?(Array) && v2.is_a?(Array)
+            v1 | v2
+          elsif [:undefined, nil, :nil].include?(v2)
+            v1
+          else
+            v2
+          end
+        end
+
+        hash1.merge(hash2, &merger)
+      end
     end
   end
 end