The Witcher
Own two boxed versions, the original Polish release (regular edition, but it has content from special editions from some other countries) and a very special Enhanced Edition. Special how? Like this. And it was comparably cheap, too. I think I paid 20€. Went on to also buy it digitally later on.

The Witcher 2
Own one boxed version, the GOG version and because I'm an achievement whore (not really, but I like them) I also bought the Steam version eventually (don't look at me like that - they motivate me to do several playthroughs in a game that really deserves them).

And more often than I can count I've bought English versions of games that I had previously owned only in German or Polish. Especially because of the censorship in German versions. I think the most important case was Half-Life which was for a long time my favourite game and which suffered from truly atrocious censorship (as well as terribad voice acting) in the German version.

I've bought new versions of many adventure games on GoG that I owned on old store-bought disks (especially Darkfall, Atlantis series, Sierra series Spacequest & King's Quest, the Myst series, etc.) I actually have some floppy disks with Zork (AND cd versions AND GoG versions lol) , Civ 1, and a few other games (unless they are more lost than I think.) So...yeah

Hmm, The Wolf Among Us one steam copy and the other one bought from telltale's website.

Though afterward I felt so stupid, why I want to bought 2 copies =.=" Just to support the developers >.<

Final Fantasy Tactics is one of the few games I'd do that for. War of the Lions really added a nice spin on it. I just love the world of Ivalice.

My older brothers had Heroes of Might and Magic 2-3 on disc, but 2 shattered and 3 has a few glitches on modern computers, so I got GOG copies of both.I'm still very proud of my Heroes 3 disc though, one of the only relics I have of the past. I wish I could have experienced the games of the 90s in their prime.

Baldur's Gate and BGII
Diablo
Diablo 2.
Realms of the Haunting
Lands of Lore III
Realms of Arkania 1, 2, and 3
Pool of Radiance
Pools of Darkness
Curse of the Azure Bonds
Gateway to the Savage Frontier
Final Fantasy IV
Final Fantasy VI
Final Fantasy I and II
Sonic
Golden Axe 1, 2, and 3
Streets of Rage 1 and 2
Phantasy Star (whole series, probably 3 or 4 different times)
Fallout 1 and 2
Fallout 3
TES III:Morrowind
TES IV:Oblivion
TES: Arena
KotOR 1 and 2
NWN 1 and 2
Gothic 1, 2, and 3
Risen
S.T.A.L.K.E.R. Shadows of Chernobyl
Doom 1, 2, Ultimate and 3
StarCraft
Warcraft 2
Freespace 1 and 2
Vampire: The Masquerade
V:TM: Bloodlines

And I'm sure there's a few I missed. Most of them are games where I own or owned the disc version but lost it through the years, or wanted a good digital backup.
EDIT: Missed Bloodrayne 1 and 2 as I had them on PS2 and now on GoG.
EDIT 2: DarkStone
Summoner

Gonna leave it at this though. as I'm afraid I'll spend the rest of the night finding these games. :P

Tropico. Have 2 disc versions and 4 digital ones. Just because I love it.

2 more to add;

Battlefield Vietnam

Got it when I was little. My older brother accidentally broke one of the disk, trying to uninstall the game, because, at the time, had a dirt cheap computer which ran the game a 1 frame per minute (or several minutes). We were going to sell it, along with Battlefield 1942. My dad had a booth at the local flea market. No one ever bought 1942, so, when I got into PC Gaming (with a much more powerful machine to boot), we retrived it from the display it was in. As for Vietnam, we had to order another copy off of Amazon.

Command & Conquer Generals

Another game I got when I was young. A friend of the family introduced it to me. At the time, like with Battlefield Vietnam, it ran at 1 frame per minute(s). When I got into PC Gaming, I retrieved it from whereever we had it stored. A couple of years ago, Wal-Mart had a copy of Command & Conquer First Decade collection. I bought it, and replaced my standalone copy with the one in the First Decade collection.

I've got the Forgotten Realms Archive Silver Edition on the way in the mail. Complete with Manual and code wheels. Not going to let it get away a third time. Hope to see some of these awesome games here soon. :P

On a slightly different note, it will be nice to play these the way they were intended again as the last time I had them, I only had Windows XP and didn't have a way to make them work right like I do now. :D Menzoberranzan is one I never got to play in its original form. Looking forward to that.

Baldur's Gate 1 and 2, and Icewind Dale 1 and 2

Operation: Flashpoint (3 times - once as the core game plus the add-ons, once as the GOTY with everything in one package, and once here on gOg)

SPAZ, just to support its arrival here even though I had it DRM-free a couple years earlier from Amazon.

Mount and Blade Warband. I have the disk version but it's loaded with Steam. There is a legitimate DRM-free workaround from Tale Worlds but I figure the gOg version already takes care of that problem so I don't have to hunt for the workaround whenever I reinstall the game. And the release-day sale price was too good to ignore.

Crusader games because gOg made them playable again. Really great games, perfect examples of why gOg does what it does.

Interstate 76

Medal of Honor, to try it out on hardware that can actually handle it (at the time, my PC could barely keep up) and also to pick up the last expansion at the same time.

Magic Carpet games, just because.

Wing Commander 1-4. Er, I think I used to own 4. Memory is fuzzy.

Privateer 2, because I was having game crash problems when it was new and have never actually finished it.

Populous, just because.

Freedom Force v 3rd Reich. To support the DRM-free existence of this fantastic game.

Darklands, for nostalgia. Also crashed a lot on me back in the day so I figured I could give it another go.

I-War2. One of my all-time favorites and there was no way I was going to pass it up.

Total Annihilation. In part to get all the patches and add-ons in one package. Great game.

Also, Master of Magic. How could I forget my entire reason for registering for GOG..?

At least a dozen games. Trine, Creatures, Disciples 2, Spellforce 2, Games from Steam when migrating... Diablo 2, Ring Runner, Baulder's Gate 1&2, Neverwinter nights, Sacred & Sacred 2.

Probably more but aren't coming immediately to mind.

Oh yeah, that one, too. Thanks for the reminder!

Some of the Wing Commander series and some early-90's games (Harpoon, Harpoon2, etc.) and that's it. And those only because I got some kind of package deal with a new version or major new content I didn't have. I tend not to repurchase things.