[BUG] was able to enter & win GA for already owned DLC

You are responsible for the giveaways you enter.

Not a bug, steam's API doesn't report DLC.
You screwed up, just ask in a comment on the giveaway for a re-roll and hide giveaways for any DLC you own.

yeah i think that's what i'll try to do, thanks @robilar5500

Our friendly members have already answered your question, but I will reiterate. Do not use the key, do not mark the game received, and do make sure you post a comment in the giveaway's comments section. In your comment, say something like, "Please re-roll this giveaway." The creator of the giveaway should make the ticket, and Support should re-roll a new winner.

Same issue for me last week (https://www.steamgifts.com/giveaway/JcmS9/starter-pack-for-arcane-preraise) and it looks like giveaway creator doesn't want to reroll as there is no answer since 1 week...
I probably entered 2 giveaways for the same DLC twice as his giveaways last usually 1 month or so. When I won't it at the 1st giveaway I forgot to check if I have other entries for the same game pending. Too bad to see this key unused while another SG member would like to use it.

Is there no way to hide all owned DLCs from view like with the games?

Is it possible to add an option that would allow us to hide all DLC for a specific game?

For example.. say I go to a game that has DLC. There can be a button there (or on the hide confirmation pop up) that will hide all DLC for that particular game as well.
OR :)
I go to a DLC for a game and when I go to hide it it I can select "Hide ALL DLC for this game"

Think that could be a handy feature.