Fallout 3 - Got the vanilla version then later got the GOTY edition with all the DLC for about £5 second hand.
Technically own lots of duplicate digital copies of games I already own from Games with Gold - you never know the disc might get scratched and free is free.
Have a few HD versions of old games - Devil May Cry, San Andreas, Metal Gear Solid, Rayman, Banjo-Kazooie and so on.
Have occasionally bought second hand games for a couple of pounds, found the disc was scratched and then not been able or bothered to return them, the cost of my bus fare and an wasted afternoon means its not worth it - Bioshock 2 and the original Assassin's Creed are examples of this.
Moderator edit: Thread locked to avoid further necro-ness.