I have 2 copies of Heroes of MIght & Magic 3 at home, but I don't remember buying it twice, so the other copy is probably my friends. I also bought Diablo 2 (& LoD) twice because I lost the CD-keys.
Weird case with my steam, my friend cheated on my acc so it got banned on VAC servers, he bought me most of the games I owned on the first account, but I had to get some myself.
And Red Faction: Guerilla, which probably doesn't really count since I first bought it for PS3, then got it as a gift alongside Metro 2023.