Entering giveaways for games already owned

If the GA is for a bundle [with base game + DLC or 2+ games], the person can still enter even if they already own part of it.
it's up to the GA creator to ask for re-roll saying the winner already own the base game or most of the bundle.

maybe he deleted the game from his account > entered > won > reactivated it > traded it. ¯\_(ツ)_/¯

send a ticket with all the info you can find and let support investigate.

Was the GA for a base game or some kind of GOTY or Complete Collection? In the latter case, SG will not properly detect it as owned and thus allow entry. GA creator has the right to reroll if the winner owns the base game or a significant part of the collection, but it's not obligatory.

the giveaway was one single game, the game is bundled however according to this page https://www.steamgifts.com/bundle-games

For the hours it could have been a game with a free weekend, or a game they played through family sharing. If it's was actually owned, though, and just a single game with no DLC I don't know how they could have entered.

There are also very few exceptions of games (most commonly games removed from the Steam Store) that SG does not always detect properly. I wouldn't assume this to be the case for something with 100+ hours though.