C# Compiler Warning 1685

Another easy way to verify:
In your code, temporarily use the class somewhere.
Example:

System.Runtime.CompilerServices.ExtensionAttribute x = null;

When building, this will generate error:

The type ‘System.Runtime.CompilerServices.ExtensionAttribute’ exists
in both ‘c:\Program Files\Reference
Assemblies\Microsoft\Framework\v3.5\System.Core.dll’ and …..

And show you immediately the 2 sources causing the conflict.