Seat duplications are extraordinarily rare. But when they happen, it’s best to let the staff work it out.This seems to happen to me all the time. Most often, someone sits one row up or down from where they are supposed to be. Those are honest mistakes, and are the best situation. Another situation involves folks who try to do a minor upgrade by moving from an assigned middle seat to, say, and aisle seat.
Then there are brazen people with an alternate set of moral rules and questionable upbringing, who deliberately self upgrade by sitting in first class when they have an economy ticket. Sometimes they are belligerent, and claim to have a ticket. The flight attendant must get involved.
My approach is to ask the person in the seat if they have a boarding pass. Since there is a non-zero possibility that the airline issued two tickets for one seat, I ask the flight attendant. If I have the rightful ticket, I will be in that seat. I let all parties know that I am not happy.
Otherwise, yeah, I’ve seen lots of folks making an honest mistake.
But once in a while, you get people, who generally don’t fly much, who don’t understand what they bought.
They never read their contract of carriage, or the terms and conditions under which they bought the ticket.
So, little things like “changing seats“ and moving from economy, to economy plus where everyone else paid more for their ticket, is a violation of that contract. It’s like ordering a regular hamburger, but trying to grab Big Mac off the shelf.
You’re entitled to what you paid for. Nothing more. If the airline gives you an upgrade out of their convenience, then you got lucky.