In NJ, as kiid well knows, everyone got a mail-in ballot, but if you went to vote in person -- which was discouraged, but allowed -- you filled out a provisional ballot, and they checked to make sure your mail-in ballot wasn't received and counted.
In practice, I'm pretty sure that meant counting all the mail-in ballots and after those were done, taking the provisional ballots and checking the vote register. It's difficult to vote twice and have both counted. Illegal as well. And one extra vote is hardly worth the risk.