diff --git a/indexing-service/src/main/java/org/apache/druid/indexing/overlord/RemoteTaskRunner.java b/indexing-service/src/main/java/org/apache/druid/indexing/overlord/RemoteTaskRunner.java
index 42ff3c246763..c3c00e2f0e6f 100644
--- a/indexing-service/src/main/java/org/apache/druid/indexing/overlord/RemoteTaskRunner.java
+++ b/indexing-service/src/main/java/org/apache/druid/indexing/overlord/RemoteTaskRunner.java
@@ -31,7 +31,6 @@
 import com.google.common.collect.Collections2;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
-import com.google.common.collect.Iterables;
 import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
 import com.google.common.util.concurrent.FutureCallback;
@@ -100,6 +99,7 @@
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Iterator;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
@@ -464,8 +464,24 @@ public RemoteTaskRunnerConfig getConfig()
   @Override
   public Collection<RemoteTaskRunnerWorkItem> getKnownTasks()
   {
-    // Racey, since there is a period of time during assignment when a task is neither pending nor running
-    return ImmutableList.copyOf(Iterables.concat(pendingTasks.values(), runningTasks.values(), completeTasks.values()));
+    // Use a map to dedupe tasks, since they may transition from one state to another while this method is iterating
+    // through the various collections.
+    final Map<String, RemoteTaskRunnerWorkItem> items = new LinkedHashMap<>();
+
+    // Racey, since there is a period of time during assignment when a task is neither pending nor running.
+    for (RemoteTaskRunnerWorkItem item : pendingTasks.values()) {
+      items.put(item.getTaskId(), item);
+    }
+
+    for (RemoteTaskRunnerWorkItem item : runningTasks.values()) {
+      items.put(item.getTaskId(), item);
+    }
+
+    for (RemoteTaskRunnerWorkItem item : completeTasks.values()) {
+      items.put(item.getTaskId(), item);
+    }
+
+    return ImmutableList.copyOf(items.values());
   }
 
   @Nullable