The thing I heard that is most laughable is that he has voted via mail! He is "allowed to" but no one else is because fraud.
That's how I understand the situation to be.
All the mail in ballots require a signature which is typically verified against the signature in DMV or one you provided during registration.
Sometimes the ballots are rejected because it doesn't match. You can actually check online if your vote was counted.
If you decide to cast a vote in person and you received ballot by mail, you can surrender your mail ballot too get in person one. If you don't provide one, you can cast a provisional one, which will be counted only of they don't receive mail in ballot, if they do you also will be in trouble.
John Olivier also had response [1] about voter fraud.