Setting EnableDefaultCompileItems=True in a file-based app results in CS2002 as app file is included multiple times

[DamianEdwards]

If you set EnableDefaultCompileItems to true via a directive in a file-based app, in order to re-enable the default compilation items (like a regular project), you end up with a CS2002 warning being raised because the main file-based app .cs file is still explicitly included in the virtual project.

1. Add #:property EnableDefaultCompileItems=True to a file-based app
2. Build or run the file-based app
3. See CS2002 emitted for the app

We should consider conditioning the explicitly added compile item so that it's not added if EnableDefaultCompileItems is already true.