Building Blazor Web App in VS fails

[vsfeedback]

This issue has been moved from a ticket on Developer Community.

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[severity:It's more difficult to complete my work] [regression] [worked-in:17.13.6]
See the reported error blow. The only thing that seems to work is: open a command prompt/powershell and manually run 'dotnet clean' and 'dotnet build' sequantially. Using the VS UI to clean, build or rebuild always results in this error.

The "ApplyCompressionNegotiation" task failed unexpectedly.
System.ArgumentException: An item with the same key has already been added.
at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
at System.Collections.Generic.Dictionary2.Insert(TKey key, TValue value, Boolean add) at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable1 source, Func2 keySelector, Func2 elementSelector, IEqualityComparer`1 comparer)
at Microsoft.AspNetCore.StaticWebAssets.Tasks.ApplyCompressionNegotiation.Execute()
at Microsoft.Build.BackEnd.TaskExecutionHost.Execute()
at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext()

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Original Comments

Feedback Bot on 5/19/2025, 07:59 AM:

We have directed your feedback to the appropriate engineering team for further evaluation. The team will review the feedback and notify you about the next steps.

[javiercn]

Thanks for contacting us.

We believe this issue has already been addressed in 9.0.301 which corresponds with Visual Studio versions 17.14 an newer.

[MihuBot]

Possible related and/or duplicate issues:

• [dotnet-sdk-9.0.100-preview.7.24407.12] Fail to build the app with error “System.ArgumentException: An item with the same key has already been added.” #57423
• [dotnet-sdk-9.0.100-preview.5.24281.15] Fail to build the app after retargeted to .NET 9.0 with error “System.ArgumentException: An item with the same key has already been added.” #56029
• [Blazor] DefineStaticWebAssetEndpoints unexpected exception #56176