You invalidate duplicates and notify the real voter they need to re-cast their ballot. You then hand off the evidence to the police for prosecution. But that

You invalidate duplicates and notify the real voter they need to re-cast their ballot. You then hand off the evidence to the police for prosecution. But that is getting ahead of ourselves. The problem isn't how do you solve duplicate voting, it is how do you detect it. That is a solved problem, already, and the answer is that there are very few attempts at voter fraud in a vote-by-mail system. Now, the electronic machines are a whole 'nother matter. I would fight hard against my state implementing such a disaster. Vote by mail is great.