I have physical cds, but I re-bought some to save time/pain on tinkering on modern systems and for backups.

Heretic - floppy to backup cd to finally gog
Doom - floppy to backup cd - 2nd cd [doom/doom2/extra cd]
Disciples II -cd-steam-gog
Master of Orion cd -gog
Master of Orion II cd -gog [old win cd quit working on newer win]
Alpha Centauri cd-gog
Planescape Torment cd-gog
Pax Imperia: Eminent Domain cd-gog
Dungeon Hack cd-gog
Dark Sun1/2 cd-gog
Menzoberranzan cd-gog
Flatou2 cd-gog
Oblivion cd-gog

There are games which I bought three times. This especially includes Heroes of Might and Magic 3. Only the English copy owned on GOG is fully bug free. Two previous boxed editions are in Polish and have some nasty flaws. Basically I only use the GOG version now. It's a very stable base for modding.

A few too many. Some games I bought three times (gog, PS4, Steam).

Quite a few actually...

The most re-bought game for me is Master of Orion 2:
- Bought it around original time of release for our family PC
- Bought a Mac version a number of years later
- Rebought a PC version due to the loss of the original copy
- Bought the digital version when it was released on GOG :)

Runners-up:
- Sid Meier's Pirates! (the remake): PC, Mac, and GOG
- Civilization 3: PC, Mac, and GOG
- Civilization 4: Mac, Steam, and GOG
- Heroes of Might & Magic 3: PC, Mac, and GOG

Other games of which I used to own physical discs and have rebought digitally on GOG - pretty much all of my physical copies were either sold, given away, or (occasionally) lost during moves:
- Star Trek Armada
- Star Trek: Elite Force 1 & 2 (Mac)
- Tomb Raider: Legend, Anniversary, Underworld (GameCube/Wii)
- Homeworld 1 & 2 (PC/Mac)
- Myst 1-4
- Bioshock (Mac)
- Bloodrayne (GameCube)
- Ankh
- Freedom Fighters (GameCube)
- deBlob (Wii)
- Warcraft 1 & 2
- Diablo
- Brothers in Arms 1 & 2 (Mac)
- Sid Meier's Railroads!
- Indiana Jones and The Last Crusade, The Fate of Atlantis, The Emperor's Tomb, and The Infernal Machine
- Monkey Island 1-3
- Lego Star Wars (Wii)
- Sphinx and the Cursed Mummy (Gamecube)
- Syberia 1 & 2
- Full Throttle
- Little King's Story (Wii)
- Tron 2.0
- Day of the Tentacle
- Spore
- Mirror's Edge (Xbox 360)
- Dead Space (Xbox 360)
- Vampire: The Masquerade - Redemption & Bloodlines
- Railroad Tycoon 3
- Galactic Civilizations 2
- Return to Castle Wolfenstein
- In Cold Blood
- System Shock 1 & 2
- Doom 1 & 2
- Eye of the Beholder (Amiga)
- Worms: Armageddon
- Massive Assault
- Cyberia
- World of Goo (Wii)
- Wings (Amiga)
- Star Wars: Jedi Knight - Jedi Outcast & Academy (Mac)
- Star Wars: KotOR (Mac)
- Star Wars: Rebel Assault
- Star Wars: X-Wing Alliance
- The Dig
- Toonstruck
- Star Wars: Dark Forces 1 & 2
- Star Wars: Empire at War (Mac)
- Sam & Max: Hit the Road
- Aliens VS Predator (Mac)
- The Longest Journey & DreamFall
- Wacky Wheels
- Broken Sword 1-3
- Runaway: A Road Adventure
- Red Faction (Mac)
- Red Faction 2 (GameCube)
- FlatOut 2 (Mac)
- Privateer 2: The Darkening
- Clive Barker's: Undying
- Magic Carpet
- Bioforge
- Duke Nukem 3D
- Prince of Persia: The Sands of Time (GameCube), Warrior Within (GameCube), The Two Thrones (PC)
- Legacy of Kain: Soul Reaver (Dreamcast)
- Pirates! GOLD
- RollerCoaster Tycoon 3 (Mac)
- Populous: The Beginning
- Heroes of Might & Magic 5 (Mac)
- Deus Ex
- Worms United
- Unreal, Unreal Tournament, and Unreal Tournament '04
- Far Cry
- Little Big Adventure
- Crusader: No Remorse
- Expendable (Dreamcast)
- Stronghold
- Beyond Good & Evil (GameCube)
- Dungeon Keeper 1 & 2
- Might & Magic 3-5
- Psychonauts (Xbox)
- Doom 3 (Xbox)
- Deus Ex: Invisible War (Xbox)
- Sid Meier's Alpha Centauri
- Descent 1 & 2
- Darkstone
- Overlord (Xbox 360)
- Dragon Age: Origins (Mac)
- Freedom Force

Rebought on GOG to replace Steam/Humble copies:
- HuniePop
- Saints Row 3
- Cave Story+
- Wolfenstein: The New Order & The New Colossus
- Prey
- Tomb Raider '13
- Batman: Arkham Asylum & Arkham City
- Bioshock Infinite
- XCOM: Enemy Unknown
- Life is Strange
- Deus Ex: Mankind Divided
- Dishonored 1 & 2
- Call of Juarez, Bound in Blood, & Gunslinger
- Fallout 3 & New Vegas
- Torchlight 1 & 2
- Shadowgrounds + Survivor

To think about how much money I spent on these. o.O Then again it stretches over a period of three decades or so.

Duke Nukem 3D. I think I bought it everywhere: Zoom, GOG, Steam (original from 3D Realms Anthology, Megaton Edition, 20th Anniversary World Tour).

I have a good bunch of games I purchased at one store (GOG), and then have for free at one or more other stores. Some of those I got from Humble also came with a Steam Key, so I guess technically I paid for that, though I also have the DRM-Free downloads.

I bought a few Tomb Raider games real cheap at Steam, that then became available real cheap here at GOG, so I did buy them twice ... if not for very much overall.

Most of what I have paid twice for, perhaps more in some cases, are games I had on disc. Most of that has been at GOG, with a very small number at Steam. I could list all those, though a part of me doesn't want to know, so I have never fully done it.

In my defense, many games I have re-bought at GOG, that I have on disc, are more complete in the GOG version, and certainly more up-to-date.

I'll list just a few series. There are many many more series and individual games.

Blood
Doom
Oddworld
Quake
Redneck Rampage
Star Wars
Tomb Raider
etc

Some I also have on consoles (PS, N64, Xbox, Wii, etc)

There are tons of games I owned on disc and got on GOG later. Can't list. Games I've purchased as gifts multiple times...

Many times:
FTL
Stardew Valley
Grim Dawn
RollerCoaster Tycoon 1/2/3 (Sadly 3 is gone from here. :()
Monster Train

A couple times:
Defender's Quest
Dungeon Keeper 1/2
Slay the Spire
Wingspan
Fell Seal
Parkitect
Megaquarium
Monster Prom
Pirates! [I prefer the early 00s version though have more play time in the 80s version]

Also:
FFX/X2 for PS2, PS3, PS4... Great games (both of them).

Jagged Alliance 2
Owned physical.. then bought on GOG... and multiple times again on GOG to giveaway as i love the Game so much.

My list would be too long, mostly repurchasing on GOG, but my biggest title would be Fallout 3, which i bought physical and then digital for PS3 and then on Steam and then on GOG.

Several - Sword of the Stars, Doom and Plants vs Zombies (the first time on a DRM free disc no longer in my possession). Incidentally, they are the games I regularly return to for another helping.

There are a bunch of them. Generally, if I really enjoyed a game I have elsewhere and it's released here I show my support with my wallet and repurchase it. A few that come to mind are:

Elder scrolls games
Dying Light
Sniper Ghost Warrior games
Mad Max
Y's games
Ultima Games
Bioshock games
Far Cry games

I know it will never happen but I would love to see the below games on GoG:

Any Final Fantasy games old or new
Skyrim (yes, again lol)
Fallout 4
Forza Horizon games
Newer Far Cry games
Any GTA
Red Dead 2
Any Borderlands FPS game

Thank you for all the replies. There's a fair gamut of responses, but most of you seem content to buy many games multiple times.
What happened? I don't use the store, but I'm somewhat curious what the competition gets up to.

It happened several times for games of which I had a physical copy (Fallout and Planescape Torment) and several time for games I already owned on other stores (About half of the former TellTale catalogue) or several games I got on various bundles.

It also happened on The Witcher that I bought again on GOG to thanks the develeper.


But, strangely enough, the game that I bought again and again and again most was... Machinarium! Don't ask me why. I don't know, I can't remember why.