They would have to look at every fake ballot, get on the computer and look up if that person had voted yet, and then scan that ballot.
And they couldn't even start doing that until all the other ballots had been counted.
And they kept it all secret?
Not if the system rejects a ballot after scan because of a duplicate. You'd be going through them pretty quick.
And if you only do it with one machine, there's only one log to get rid of.
I'm not claiming to "know" what happened. I can only try to figure how I would design a cheat if I wanted to.
My personal belief - was there cheating? Yes, like there is in every election.
Was it enough to make a diff? - usually it isn't but my suspicions are aroused when the counting stopped concurrently in multiple states. This implies coordination of multiple elections offices. Such a scheme could make a big difference.
Is that paranoid - absolutely not in this case. We saw 4 years of false accusation and investigation against Trump from WITHIN the government. It's a simple extension to figure these same types of folks(Trump deranged) would cheat an election and sleep well at night.