Note that most of these games have been bought twice because they were either: A) Better on PC (mods, graphics ect) and B) Recently sold my XBox to concentrate fully on PC gaming. I don't just buy games twice without thinking about it, I make sure selling the first copy can help cover the price of the second.
-Oblivion Twice (one on Xbox, then sold it and got it on PC)
-L4D2 Twice (one on XBox, then sudden Steam sale)
-Portal Twice (one on XBox, then sudden Steam sale)
-Portal 2 Twice (one on XBox, then sudden Steam sale)
-Half Life 2, 2ep1 and 2ep2 Twice (one on XBox, then sudden Steam sale)
-Fallout New Vegas Twice (one on XBox, then sudden Steam sale)
-Limbo Twice (one on XBox, then sudden Steam sale)
-Mass Effect Thrice (one part of PC Trilogy edition, other two on XBox)
-Mass Effect 2 Twice (one part of PC Trilogy edition, other one on XBox)
-Mass Effect 3 Twice (one part of PC Trilogy edition, other one on XBox)
-Rayman 3 Twice (one on PS2, other HD edition)
-Alan Wake Collectors edition twice (one the xbox exclusive pre-order, other a steam sale)
-Alan Wake's American Nightmare (one on XBox, then sudden Steam sale)
-Assassin's Creed 2 (one on XBox, then sudden Steam sale)
There's probably more, but I can't remember. Most of these games were bought because they were 75% off or something on the steam sale, and I could sell my origal copy to cover costs.
By the time I sold my XBox, all the games I cared about were on my PC and any that weren't were either console exclusives, not a part of a steam sale or simply not worth buying again.