Merge pull request #26499 from dotnet/merges/dev15.7.x-to-dev15.7.x-vs-deps

Merge dev15.7.x to dev15.7.x-vs-deps

src/VisualStudio/Core/Def/Implementation/ProjectSystem/AbstractProject.cs

@@ -624,6 +624,13 @@ private void AddMetadataReferenceCore(VisualStudioMetadataReference reference)
         {
             lock (_gate)
             {
+                if (_metadataReferences.Contains(r => StringComparer.OrdinalIgnoreCase.Equals(r.FilePath, reference.FilePath)))
+                {
+                    // TODO: Added in order to diagnose why duplicate references get added to the project. See https://github.com/dotnet/roslyn/issues/26437
+                    FatalError.ReportWithoutCrash(new InvalidOperationException($"Reference with path '{reference.FilePath}' already exists in project '{DisplayName}'."));
+                    return;
+                }
+
                 _metadataReferences.Add(reference);
             }