Two output file names resolved to the same output

This can happen in the event of two .resx files pointing to the same form. Mostly happens when renaming forms (other reasons may apply, I’m not exactly sure)

If your particular form files looks like this:

Form1.cs
Form1.designer.cs
MyFormerFormName.resx
Form1.resx

then that usually implies that you renamed the form but Visual Studio didn’t remove the old .resx file. Once you delete the file (in this example MyFormerFormName.resx) manually, the error should be gone upon next build.