I ran into this bug today which was unfortunately closed as can't repro. I was renaming some AppSettings variable names (right click on a Project's Properties and go to the Settings tabs) and deleting some other settings, when I did a Search/Replace for the entire solution and somehow wound up with a duplicate Settings.Designer.cs file named Settings1.Designer.cs. This was effectively a clone of Settings.Designer.cs which meant:
The fix was to
You should be good to go, if you get an error about a missing file, you may need to manually open your csproj or sln file in Notepad and ensure that the MSBuild script doesn't contain a link to Settings1.Designer.cs.