Bizarre situation 6/8 AS1426 SEA-LAS. Two passengers assigned 4F. FA attempts to sort it out with the handheld. Passenger 1 (seated) does not have his boarding pass and is not the name on the handheld, so asked to return to the gate agent (looks not happy) while assigned passenger 2 takes the seat. Several minutes later, gate agent arrives and asks passenger 2 to deplane. Several minutes later, passenger 2 returns and says a “glitch” occurred and both passengers assigned 4F. Passenger 2 has his UG canceled. As passenger 2 tells it while moving his stuffs to PE, passenger 1 apparently originally had the seat but never checked in and his tix was canceled. Not sure how he boarded (or made it by security) without checking in or a boarding pass??? Original 4F seated passenger 1 returns and takes the seat. He does not offer commentary.