ok, but how do you overbook a flight without booking at least one of the seats twice?
Sometimes it doesn't work out and that's when airlines bleed. I was flying home one day from some US city (was it Denver? the years and cities blend together) when a Drupal conference and some major sports thing ended the same day and the airport was a total gong show. United first offered a paltry 120 USD but when they raised to 400 USD to go home the next day instead my hand went up -- and I was such a greenhorn I didn't know they will pay for the hotel, too. A buddy pocketed no less than 1200 USD from Delta.