More Trump voters now see this as an option, so Democrats have to put a way for it not to happen. The only ways to do that are either make a push to punish voters who both mail in and vote in person with voter fraud (which is an easy equal protections suit), or make it not against the law. The way to make it not against the law, is to count ONE VOTE PER PERSON, either by not counting those whose ballots don't match, or PRIORITIZING IN PERSON VOTES.