... or the stowaway has found an empty seat (not likely nowadays).

1) Give the ticket to the first person in the check point, get scaned, talk to the their children that is makin fuzz, give the ticket to the second person in the check point, alarm, nobody remember that they gave the ticket to the first person because everyone was in autopilot.

2) Give the ticket to the first person in the check point, get scaned, notice that the partner went to the bathroom, wait, give the ticket again to the first person in the check point, alarm, nobody remember that they gave the ticket to the first person because everyone was in autopilot.

I bet passengers do this often enough.

You're traveling as a family. You print boarding passes and distribute them to everybody. Somebody loses theirs. So you print new copies, but you don't necessarily give people the same seat number as the first time.

When boarding the plane, one of you reaches into their bag and grabs the one they thought they had lost. So two of your family members are using the same pass, and one is going unused.

No harm is caused because the number of passengers on the plane is still as expected, and the family is able to figure out for themselves who takes which seat.

I was just recently traveling with someone and they had a copy of both of our boarding passes (I just had mine). I boarded first and heard behind me some issue happening: they'd accidentally given my pass so it popped up as already boarded. They just switched to the right pass and there was no issue. It would have been a little more complicated if we had just happened to board in the opposite order, though.

So, I'm sure the general issue of "this pass was used already" happens a lot. How often it gets resolved without the would-be passenger presenting a "good" one, who knows.