Years ago I recall boarding flights and literally there would be a situation whereby someobody was already sitting in your seat with a valid boarding pass. Luckily this never happened to me. However I recall arguments with disgruntled passengers and sometimes if there were no spare seats you would hear the cabin crew over the loudspeaker offering passengers money to disembark. I wondered, Does this still happen or are there new systemns in place in recent years to ensure this cant happen.
I think that these days with all the IT etc this sort of thing happens at check-in rather than on board - even more so because of the baggage security issues.
It's easier to control 'off-plane' in terms of handling the pax etc