I hardly ever buy a game more than once usually, at most sometimes twice for some games, but not on the same system. For example, I bought Call of Duty 4: Modern Warfare on PS3 and then the remaster on PS4. Same with Bionic Commando, I bought that on PS3 and then years later bought it again on Steam. Also Resident Evil 4 where I bought the game on PS2 and then bought the HD version of it on Steam much later on. 

The only exception to that where I can remember buying a game a lot of times is Red Faction: Guerrilla. Bought it in 2009 on PS3, bought it again on PS3 in 2011 but took it back after a few days, then bought it again on PS3 in 2012. Then I bought it on Steam at some point in the past few years. I might buy the new remastered version of it when it comes out as well, or as they call it the reMARStered version, lol.

So i've bought that game 4 times as of right now, 3 times on PS3 and 1 time on Steam, and might buy it a 5th time when the new version of it comes out. Safe to say that I really love that game, lol.

I do this if:
1. I like the game a lot. Example: got Overcooked for X1, PS4 and Switch
2. I like the game a lot and play with different friends. Example: got Rocket League for X1, PS4, Switch and Steam
3. I like the game a lot and preserve a copy in case it breaks some day
4. I think the game might be worth more sometime in the future (not like a few days/weeks but years/decades)
5. I own a PAL copy of the era where Hz mattered and get the NTSC-J copy for the true feeling

It's very rare. Even with remasters it's rare because I keep my older games and consoles. I do buy every Zelda release, so for example I have BotW on both WiiU and Switch.

Rarely, but some games are just so good, so I buy them more than once, both for convenience and for supporting the developer.

Buying the same game multiple times seems incredibly wasteful to me. I can't imagine doing it, except in a very rare circumstance where the same game spans multiple generations and you're playing it when you upgrade.

The more I see this thread on the Hot Topics, the more and more I kick myself in the ass for not returning the duplicate Borderlands Handsome Collection copy back to Amazon.

I pre-ordered it digitally, then had a moment to buy it before release so i did. Then i bought the ultimate collectors edition later, and then i bought the royal Ed 

I buy duplicates of many games (especially favorites), mostly to always have the most current version, because I never want to find myself in a situation where I'm without the ability to play a favored title. For example, of late I've been amassing a number of titles for Switch that I have on PlayStation 4 already, including Crypt of the NecroDancer, Night in the Woods, The Flame in the Flood, and Ittle Dew 2+ (which counts as an updated version more than a port really). Planning on getting the Switch version of Kentucky Route Zero as well. It's just awesome to be able to take those sorts of games anywhere I go and that's a real advantage of the Switch! I'd much rather play those on my lunch or dinner break than 3DS fare.

Do you have any idea how many versions of the original Super Mario Bros. I have? It's a lot!

I most especially like to get multiple versions of rare games. For example, I originally got a fan-dubbed Nintendo 64 cartridge of Wonder Project J2, but more recently have downloaded the computer version because I'm worried about the age of the cartridge and its ability to retain save data going forward at this point.

You get the idea. I'm a pack rat with games. I don't like the prospect of ever losing anything I have, so I typically try to get new versions of stuff I like when they come out.

Wouldn't it be cheaper to replace the disc drive and get all your retail versions working again?

Nope, one version for me. Even remakes or remasters for games I already have depend greatly on what game it is, and it is rare that I go through with it.