Though it was a few years ago, as someone who has worked the polls, here is what I recall about the poll operation and ballot collecting process:

In MA, absentee/early ballots are sent out with barcodes that can be mapped back to a central DB. Afaik a maximum of one early ballot is sent out to any given voter. If you lose it, you’re walking to your polling station on election day. The mailed/dropped off ballots are scanned so that they’re routed to the correct voting precinct; they’re given to us on Election Day to run through with the in-person ballots.

On Election Day, we sit there with registered voter lists (the list is sharded, not duplicated - i.e. someone works a-f, someone else works g-k, etc.); people come in, tell us who they are (no, no id is checked, but they do need to give us a full, correct address that matches their stated name); they are marked off as “voted” on the list. Voters are provided with a ballot once they have been found on the list and marked off.

Absentee ballots are run through the machines throughout the day; for every single ballot, we match it to a name on the list, and . If there are any duplicate ballots, that is caught at the voter list checking phase, and is flagged thusly for any necessary follow-up (confirmation, disambiguation, or legal action as necessary) and the extra ballot is set aside (whichever one we come across second).

I’m pretty sure we had zero duplicates when I did it in the 2020 presidential (I feel like I would have remembered that, considering the political context at the time), and iirc we processed something in the neighborhood of 10,000ish ballots.