Basically nothing. If you wanted to show up to a polling station and claim to be your neighbor to steal their vote, there is really nothing stopping you. Except that, yaknow, you would have to get off your ass and go do it. Which is why voter fraud is essentially a non-issue. The number of fraudulent votes any given person could cast would be negligible in terms of the total ballots cast in an election. So the individual has little chance of swaying the election, and knows it. So there is no benefit for them. And in return for no benefit, they would need to leave their house and essentially do paperwork, which is annoying and boring.

Like, if you're going to go to the trouble of committing a federal crime, you might as well commit a profitable crime.

So we have specific polling locations we have to vote at, and in my state there’s literally a book with a list of names of everyone who will vote that day. They mark the book as you come in. So, if I vote first, the other person will be immediately caught. If the other person votes first, then it will get complicated and depend on your local rules. I would guess I will have to show ID, talk to police, and do a special ballot, but they don’t really make these procedures public. Because voting fraud is extraordinarily rare. Also the person would need to go to the correct polling location, which there are dozens of in my city. The odds of a scammer doing that correctly are low.