The "ResolvePackageDependencies" task failed unexpectedly - An item with the same key has already been added

[jogibear9988]

Error MSB4018 The "ResolvePackageDependencies" 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.NET.Build.Tasks.ResolvePackageDependencies.GetPackageAndFileDependencies(LockFileTarget target)
at Microsoft.NET.Build.Tasks.ResolvePackageDependencies.RaiseLockFileTargets()
at Microsoft.NET.Build.Tasks.ResolvePackageDependencies.ExecuteCore()
at Microsoft.NET.Build.Tasks.TaskBase.Execute()
at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
at Microsoft.Build.BackEnd.TaskBuilder.d__26.MoveNext()

Details about Problem

VS UI

Detailed repro steps so we can see the same problem

1. I have a complex Solution. This error I get after some Nuget Updates during build.

2. How can I find out wich Nuget is causing the issue and why?

[dominoFire]

• Are you using a lock file?
• A way for diagnosing the failure is to create a binary log and analyze it with MSBuild Structured Log Viewer.

Please, fill all bug details in the form (nuget.exe version, dotnet.exe version, Windows version, VS version, ...)

[zkat]

This doesn't seem like a bug in our own codebase, but @dominoFire's request above should give us a better clue of where to take this :)

[jogibear9988]

This repository shows the issue:
https://github.com/jogibear9988/nugetIssue

[jogibear9988]

Maybe it's an error in some of the nugets, but the error message in visual studio does not help.
We had a huge solution, and I had to remove more and more projects and code files to find the root cause.