I started a topic on the NW forum about this. Simply, on a 3 leg flight on NW, my GF used the BP for her 3rd flight on her 2nd flight. Since she had the same seat assignment for both, the computer didn't catch it (BPs were scanned). The pax name nor flight doesn't seem to be relevant otherwise the computer would reject the BP at the gate. I think only if 2 people are assigned the same seat will it flag something.