Quick rundown from a poll worker on the safeguards against people voting by mail and in-person. Obvious caveat that elections are generally run at the county or even municipal level, and things vary wildly.

So what would happen if you tried to vote twice? 1/x https://twitter.com/maggieNYT/status/1301286928545964034

Each precinct gets a book of every registered voter. When you come in to vote, poll workers verify that you are in the book. If you're in the book, you sign by your name and are given a ballot. 2/x

But what if you'd also voted absentee? Well, absentee ballots here are counted at the precinct level; poll workers bring the sealed envelopes up, we verify the name and address match what's in the book, and then we write down ABSENTEE next to that voter's name. 3/x

If you try to vote in person after we've tabulated your absentee ballot, you'll be reminded you voted absentee and told to leave. This happened to a voter in the last election, where he had forgotten he'd already voted by mail. 4/x

But what if you vote in person, and then we discover you've also submitted an absentee ballot? Well, as part of the check-in process, we ask every in-person voter whether they've submitted already an absentee ballot, so hopefully it shouldn't happen. 5/x

But let's say it does. Well, when we go to mark you down as having voted absentee, we would see that you've already signed the book! We'd set your ballot aside, write up a report, and if you had done this purposefully, that's criminal voter fraud. 6/x

So yeah, I wouldn't recommend trying to vote twice. Under no circumstances would both your votes actually be counted, and it's both (1) very illegal and (2) very obvious what you've done. 7/7