A while ago i got The Elder Scrolls4: Oblivion in a giveaway. It shows in my library as:
The Elder Scrolls IV: Oblivion - Game of the Year Edition Deluxe
(game id 1458058109)

When i browse the current sale and check "hide all owned products" it still shows The Elder Scrolls 4: oblivion in the list and allows me to buy it for 7.99

I must be missing something, but it looks like the exact same game to me. Am i overlooking something or is this an error on gog's side?

There are two entries for Oblivion in GOG's database.

Oblivion the package
Oblivion the Game

"Oblivion the package" is a bundle containing "Oblivion the game" and the freebies Arena and Daggerfall.

You probably only own "Oblivion the game". The store only sells "Oblivion the package". and doesn't have the sense to realise you may already own everything within the package.

This has been a limitation of GOG's store for years, and apparently is a hard problem to fix.

Yes. I'm getting the same thing, even on the store page itself. Arena and Daggerfall are automatically added to user accounts with when buying any Bethesda title. Presumably this also applies to getting Oblivion via Amazon Prime, though I got them from an earlier purchase, so this is a moot point. This is a particularly daft example of that, as there's no visible difference to the user.