Code Alert! - If you get this error:

The item "SomethingSexy.cs" was specified more than once in the "Sources" parameter. Duplicate items are not supported by the "Sources" parameter.

If you do some crazy ass copying of cs files, sometimes it fucks up the sources parameter in your csproj file. To fix, just open up your csproj file in notepad or a better alternative and under the Project\PropertyGroup\ItemGroup\Compile node "SomethingSexy.cs" will be referenced twice. Just delete one of them and the world is your oyster. Or delete more stuff and break your project. either way....