Overbooking is bullshit they do to get your fucking money so that you don't book a flight at the time you actually need to leave on a different airline.
"Well sir, we accidentally sold too many tickets [because that's fucking possible] so here are your options
•Stay here forever
•Leave later on our airline
•Take your money back and try to get a new flight, oh but it costs a fucking fortune to buy a last minute ticket.
Sorry for the inconvenience."
They booked two people on the same seat on the plane