They check whether someone has voted in person before opening and counting their absentee ballot. At least in Massachusetts, absentee ballots go in nested envelopes: an anonymous outer envelope, containing another envelope labeled with the voter's name and address. So, they open the outer envelope, and check whether the ballot is from someone who voted in person before opening the inner envelope.

That's useful not only against attempted fraud, but simple absent-mindedness: if I don't remember whether I received and returned an absentee ballot, I can vote in person. If they'd already received and checked in my absentee ballot, the in-person location would have turned me away.