Not unknowingly. But I have re-bought many games here on GOG that I do own on CD/DVD or floppy.

but why?

Sadly Galaxy could not prevent me for completely forgetting I already had the Raven Remastered on Humble Store. But most games that I've bought more than once, I've done so to support GOG.
Anyway since I use Galaxy it's easier to keep track and I don't get all the hate.

I purchased Limbo on GOG and realised months later that it was one of the Epic freebies.

It was at a deep discount, so no great loss.

Because many of them don't run or even install on modern systems and GOG offered the service of making them compatible with modern OS. Sure, I can fiddle around with dosbox myself, but games from the Win95 era are a pain in the ass and I happily paid GOG to do the work for me.

^This. Also many old games were only available in German in stores back then, and often the English version is either the native one, or at least better translated/voiced than the German (if it's not a native German game).

Sadly not here, but the German version of FF7 was not translated from the original Japanese, but from the English version, afaik. So things got lost in translation twice. The English translation was bad already, the German version was abysmal (and often broken).

I knowingly had 3 copies of Morrowind...

1 for Xbox, one for PC, and a second for Xbox when i was overseas...

Why to re-buy games?

Often...newer versions and/or re-masters might have less hassle to get to running it; more fixes in newer versions (often on GOG versions); and/or there's a version around w/ NO DRM and NO client-app's required (often on GOG versions).

XIII.
I have this for retail on the disc; and it would not install off the discs on W7 period.
Bought it off GOG for $2 - and that solved that problem entirely.
It's all I needed - as I did the rest myself with fixing: i.e. installed a fan-patch and then added DGVoodoo & config'ed that to get the game running properly at 1080p60fps full-screened with no issues.

Saboteur.
Own on Origin and also on disc from retail.
That game does not render the map properly at high-res's, in the 2 above listed versions.
GOG version has that fixed out-the-box, so...I did buy that version.

Vampire: The Masquerade - Bloodlines
My favorite game of all-time. Own it from Retail on discs (comes on 3 discs); and later re-bought it on GOG.
Steam version DOES NOT come modded w/ Unofficial Patch, so...never re-bought it there.
GOG version comes equipped w/ normally newest version of Wesp's Unofficial Patch: Basic version.
That's just one easy install and set-up without any modding needed, if I want to run it basic and cleanly.

DRM-laced games and/or games with client-app required
There are games I would probably re-buy, if they hit GOG. Games like Mad Max on Steam and Batman: AK on Steam - both have Denuvo; I have those versions. Epic version of Batman: AK doesn't have Denuvo and Mad Max on Origin also doesn't have Denuvo - so, I'd probably be looking for those, if they ever hit GOG.

Tried to a couple of times, but GOG spotted it and stopped me ^_^

Uhh... what?

Also the website tells you if you owned the game or not as well. No I never bought a game that I owned by accident. But I did do some double dipping to support the developers of said game. I bought the game first on Steam and then GOG got a release later on so I purchased the game again.

I got The Wilting Amaranth on JAST after forgetting I had it on Steam, but that is because I do not buy VNs very often. I do not knowingly re-purchase games very often either; the only times I did was to buy Vampyr, Fallout and Elder Scrolls games on GOG.