FlyerTalk Forums - Concerning situation

Here's one for you.

As I'm sitting in my 11F today for my flight from DAL to CLT on USAir this afternoon, another pax asks me if I'm in the right seat, as his boarding pass shows 11F. I dig mine out, and sure enough, we have the same seat. I suggest that he check with the FA, as I'm already settled in, a little disgruntled that I didn't get my upgrade.
He doesn't, but just settles into 11D. About 2 minutes later, the gate agent comes down the aisle and asks if I might have 2 boarding passes. I reply that I do, 1 for this flight, and 1 for my connection. He states that the computer showed printing out 2 for this leg. I reply that I really don't know, but while he is here, there seem to be two pax assigned to 11F. He looks at my BP, then that of the other fellow, and guess what? The other guy has a duplicate of mine. Not his name or seat, but mine! The gate agent starts asking questions of the other fellow, who does not speak much English, and as it turns out, he is not even supposed to be on this flight. He is booked on the next one, out of the same gate. He is escorted off, and we leave.
As we fly to CLT, I think about the whole thing, and can't come up with a reasonable scenario which would allow this to happen.

1. My boarding pass prints twice, at the e-machine, although I picked up the only BPs that I see, not only for this flight, but for my connection, as well.
2. The other guy uses the machine after I do, and either he picks up the second BP or the machine spits out the second pass at that time (so what happened to his BP?)
3. The screener at security let him through without a valid BP (all he had was my duplicate).
4. The gate agent let him on the plane without the proper BP.

Obviously I, along with everyone who had witnessed the incident were scratching our heads, and were a bit uneasy, as the guy is now no longer on the plane, but how did he get there in the first place?

By the way, this isn't a riddle, it really happened this afternoon.