Knights of the Old Republic; I already have too, but only because it wouldn't work on my PC (still doesn't ARGH!); bought a disk copy about four years ago and then a Steam copy in the January sales and neither is compatible with my PC annoyingly. No idea how to fix it so meh, at least I didn't pay much for my second copy.
I may consider buying Rome: Total War plus its expansion again because my disk copies seem to have knackered; when I try to play the game it says some file is missing from the disk and I should reinstall it; did that and still not working...
I definitely consider buying Fallout: New Vegas or Oblivion again; currently I only have them on the 360, but I wouldn't mind having them on Steam or such.
Edit: Actually, I just realised I also bought Total Annihilation twice; once from a friend for about £5 a couple of years after it came out and then for 99p from Gamestation a couple of years ago; best. 99p. purchase. ever.