So what will stop a person who voted by mail to show up at a polling place and vote there also?
Once you are sent a mail in ballot you are blocked from voting in person. If you show up, you are given a provisional ballot
How does a poll worker know you've already voted by mail? Millions of mail in votes are not likely to be counted by election day.
What are the specific safeguards?
Once they receive the ballot back, the state's have a process of checks....like in Michigan, I had read recently, they check for signature matches, check a list of those who have died from the social Security Death index, check to see if they had become a felon or moved...from the time they were mailed the ballot to receiving the ballot back....
Once all that is okay, the ballot is removed from the signature envelope and prepared in stacks, to be counted, at the proper time per their election law, and simultaneously removed or flagged on the in person voting list....made in person vote-ineligible