Build Warning: The file 'filename.ext' could not be added to the project...

I recently encountered a build warning (C# project, Visual Studio 2005) that was less than informative and somewhat difficult to resolve:

Warning: The file 'keyfile.snk' could not be added to the project. A file with the same path already exists in the project. MyProject.UI

(Note: keyfile.snk and MyProject.UI can be any file 0r project.)

Other than the terse warning, there was nothing to go on. I tried deleting the file from the OS, removing/re-adding to the project, turning off signing, etc. Nothing worked, the error persisted.

I finally opened up the offending project file (MyProject.UI.csproj in this case) in Notepad, and searched for this particular file (keyfile.snk). The source of the warning became immediately obvious -- within one of the <ItemGroup> blocks, I noticed the following:

...
<None Include="keyfile.snk" />
<None Include="keyfile.snk" />
...

So, even though the file was only being included/referenced/visible once in the solution explorer, it was actually being included twice in the project file. I have no idea how/why that happened.

Regardless, the solution is simple -- remove one of the lines, save, and reload your project.