I don't really collect games, but I have ended up with multiple copies of a few games.
RE4 - GC(LE), PS2(LE), and Wii. Even after my disclaimer, I still need to get a PC copy. I'd play through it atleast once.
RE1 - PSX, DS, and GC(Rebirth). None of them are straight ports of the other(well, DS does include one).
MGS - PSX, Bleemcast, Twin Snakes. Again, just listing together for the fun of it.
MGS3 - SE and Subsistance.
Castlevanis:SoTN - PSX(OG), XBLA, PSP.
Sonic 2, 3, and S&K - Genesis and Mega Collection.
SF2 - SF Anniversary Collection(PS2), HDRemix.
DMC3 - DA and SE.
Tetris - NES, DS.
Controllers. Umm, I probably had more N64 controllers than anything else because they broke so often. Currently, though, I have four GC controllers, so that.