First of all, when the sanctity and integrity of mail-in votes has been the #1 or #2 or #3 headline continuously for nearly 2 years; nobody, not Steven Hawking and the entire staff of NASA could have ever suspected that a mail-in voter might physically show up to cast a duplicate vote. Not in a million years. Never happen.
Second, a rule change on election day?
Third, whom shall be authorized to cancel a previously mailed-in vote upon finding that an in-person voter had already voted?
Fourth: And what if the mail-in vote and the in-person vote do not agree? Which one prevails? The first received or the in-person vote?
That's NOTHING! Just wait until the Democrat/RINO Uniparty starts making last-minute, post-election rule changes that are totally retroactive to before the election. (Coming soon to a theater near you.)
For example, all socialist/communist Democrat/RINO-run big cities retroactively make election rule changes that switch all votes for Republican candidates to their Democrat challengers, and vice versa (for this election only). I wouldn't put it past them. After all, the Democrat/RINO Uniparty easily pushed the 2020 Election date way past the actual Election Day, and the Democrat/RINO Uniparty also successfully stole the 2020 Election, so they figure there's literally nothing they can't do when it comes to elections.
Don't be surprised when 82-year-old senile Sleepy Joe puppet Manchurian Candidate gets 98.6% of the vote in 2024 and Donald Trump Jr. only gets 1.4%. (That's the kind of landslide vote margin with which Putin usually wins.)