Is there a setting so that FormReturn alerts me if someone has filled in 2 (or more) answers for the same question?

Yes, you can use a mark aggregation rule to do it. For example with 2 or more answers:


That says if B (and only B selected) is the correct answer, add one mark, if two answers are given, add 99999, for everything else do nothing.

Looking at the Form Score column in the captured data should quickly alert you to a problem that needs manual reprocessing.