I have one double booked event due to my error. I grabbed the wrong time of the same event. Despite 3 attempts to return the ticket, it was not taken care of. So now I have an unneeded ticket. I would like someone to get some use out of it. I'd suggest a board where you can place unneeded tickets, perhaps in an envelope labeled Thursday Games, Friday Games, etc. That way, people that were unable to register can play them.
I have a few of those too, but it's not really an issue now that pre-reg has closed. Anyone who wants to play your double-booked event can simply show up at game time. If a ticket holder doesn't show, his seat will be available for walk-ups.