Another (more simple) consideration to add to the others so far.

Due to a security audit, our IT guy removed Visual Studio components - which seemed to break the loading of my solution / VS marked most of my projects with a ' (incompatible)' suffix.

I finally got to the stage of only one project not building (- in relation to this question).

So I renamed the 'obj' & 'bin' directories/folders, giving them both an '_Org' suffix, and kicked-off a rebuild; but that seemed to be the issue - in my specific case, it must have been picking up the duplicate '.NETCoreApp,Version=v8.0.AssemblyAttributes.cs' file, so I deleted (/moved) both of the '_Org' directories/folders, performed a rebuild, and the error disappeared.