I had even more confusing one back in May - I got all the way on to the aircraft (777 heading to Houston) with both a printed and digital boarding pass showing I was in 1A, only to find another passenger already sitting in 1A - and she also had a printed boarding pass for that seat.
Crew didn't seemed surprised, and commented that it happens 'often' on that aircraft - luckily they were able to find me another business class seat.