Typically performed by someone who is not loyal. They will constantly move around looking for the 'next best thing'.

If you date an overlapper, you need to constant look over your shoulder. It doesn't matter how much you tell them that you love them, they will immediately and without warning start dating someone else. If they are a coward, they will break up with you and give you a completely different reason for the break up (e.g. they will lie and not admit they are breaking up because they found someone else). If they are particularly selfish, they will deny you had a relationship at all.

Got the covid-19 vaccine

NOT cheating, technically. When you start dating someone new before you've actually splitsville with your old significant other.