Dup seats do happen. I think its rarer than it used to be, though, maybe the computers are getting better.

My understanding is that the "tiebreak" is supposed to be what the computer thinks. The last couple of times I've seen it, the FA has taken both BP's out to have the GA see who should be where.

Not sure if that happened in your case. But ultimately, if the FA tells you to switch seats, you have to.