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.