In normal times you don't have to have election officials say "please don't listen to the presidents statements...it is illlegal to vote twice" but here we are...
You mean you can actually vote twice and not go to prison?You must not be very smart. It works like this... when a mail in ballot is sent to a voter, it is updated on the rolls. If I show up at a polling place, they will see that you have a ballot already. If you proceed to vote in person, they will invalidate the mail in ballot. States have been doing this for years. It is easy.