Mail-in ballots tend to be counted (and received) after in person ballots, so you don't need to worry about in-person conflict. If we go the other direction (mail-in ballot rejected because the person had already voted), it was indeed in the tens of thousands. In 2024 about 584k ballots were rejected. [1] 11% of those, more than 64k, were because the person had already voted.

[1] - https://ballotpedia.org/Election_results,_2024:_Analysis_of_...