If you’ve upgraded it form v8 to v13, could it have 2 dlls for the Website still in the bin folder? Since the v8 → v9 requires you to have a new project and then update the database, the dll files might have been copied over and the application reads both and see your duplicate. Are you seeing 2 versions of the Web project/project containing the controller?
If you completely clean the bin folder out and build again, are you still getting the same issue?