This seems to be the default response whenever there is a seat duplication. I find that my exit row seat is "stolen" at least half the time and the crew can rarely be bothered to sort it out unless I threaten to escalate the matter.

My first response to this is "perhaps they can go sit in their own seat instead". This usually gets a grumbling response. If that doesn't work, then I ask them to call the captain and advise him to start the process to offload my bags as I won't be traveling today. That has never failed to scare them into action.