I actually double booked myself by mistake one time. I was booking over a month of business travel in one night, and I just screwed up and booked the same flight combo. I didn't notice til outside the 24 hour cancellation window, so then I called just to see if there was anything I could do. Even though the ticket was unrefundable and it was 100% my fault, the phone agent just canceled one of the tickets and gave me a full refund, including all fees. I was stunned that she did that, and I definitely don't recommend expecting that, but it's worth noting that sometimes you can get lucky.