I think Ultimate Marvel vs. Capcom 3 wins it. I got it on 360, then got it on PS3 to play it with friends, and later bought it again on PS4 just to mess around with it in training mode without hooking up an older console to do it. I don't think I've ever bought a single other game more than twice.
I won a local tournament for it though, which kinda paid for two of those instances.
I don't often buy games more than once, but other instances that I can think of...
-Yakuza 0, bought it physically for $20, but my PS4 broke (only the disc drive, it plays digital games fine) and I really wanted to play the thing, and it was on a PSN sale for $5, so I just grabbed it for the hell of it to finally play it.
-I did buy Skyrim twice, once on the original 360 release and then one of the recent versions on a PSN sale to replay it/finally play the DLC stuff that I never did.
-I bought Dragon Age Inquisition twice, once on the PS3 for really cheap, and then again on PC to play it without the absurd jank of the PS3 version, and also to try the DLC.
-I own Valkyria Chronicles 1 & 4 on the PS3/4 respectively, but I bought a double pack with the two of them together on Switch recently just to revisit them + play in handheld mode. Fun games to revisit.
-I own Stardew on PC, but I bought it again on Switch for like a dollar to play in handheld mode.
-I recently rebought FF7/8/9/X-2 to revisit all of them since it's easier than hunting down my PS2 to do it, plus handheld mode on Switch.
https://www.youtube.com/user/dotsdfe
I make Fire Emblem videos. Sometimes.