I had the same thing happen to me on the LHR to MAN route last year. But the difference for me was that in my case I had a regular boarding pass , as did the passenger who had been allocated the same seat as me. I'd done OLCI and printed my boarding pass using one of the machines at LHR, not sure what the other passenger had done.

Luckily there was room for me to move to another seat. I didn't bother to ask any of the staff about this error, but I was surprised that it could happen.