The file 'file' could not be added to the project. <reason>
A file that has been read from the .vbproj or .csproj file cannot be added to the project. Reasons include:
Invalid file name.
File within a path. For example, if you have a project-relative path of File1\File2.txt, but there is also a folder with relative path File1\File2.txt.
Item already exists. This occurs when a file is listed twice in the project file.
Link already exists. The Visual Basic and Visual C# project system has a limitation in that there can only be one link with the same name per project. This means, for example, that you cannot have a link to file.vb in both folder A and folder B of the project.
This error is most likely caused by editing the project file by hand.
Any file for which this diagnostic is displayed will not be added to the project.