Sounds to me that this is a form of shenanigans, even if not the classic version. SOmeone was on the plane that the FA attendant knew that wanted 12A. Rather than have you go to the seat and compare boarding passes to see who was in the wrong seat, she told her friend to take another seat somewhere and move to 12A when the door closed. SHe asks you to sit somewhere else. You comply and friend takes your seat and FA gives you BS about duplicate boarding passes which you never even saw. Somewhat less bad version.