Every now and then this thread pops up, and I like reading everyone's reasoning behind their ideas.
Personally, I will totally buy a game again if I already own it, but as many people have pointed out, certain conditions must be met.
1) Does my old system still work/still hooked up? If no, then that's a point in favor of the game being bought again.
2) Did I love this game? Not like, not enjoy, I mean did I drown my life in this game and talked about it--and still talk about it--fondly with friends and strangers? If yes, another point.
3) Trophies? Yes, I am a trophy hunter, and I'm proud of it.
4) Does it have a game I couldn't get before, or new content? Another point.
A few games I've bought, despite already owning a copy, are:
Final Fantasy VII--PS4
Final Fantasy X/X-2 PS3
Resident Evil 4 PS3
Dark Souls II PS4
Kingdom Hearts Final Mix and Final Mix 2
Tales of Symphonia
There are others, but those are the ones that spring to mind without me really thinking.