Hello Ezzypro2.
You were able to enter the giveaway because Steamgifts cannot detect DLCs/packages as owned. This is a problem with how Steam API reports ownership and unfortunately there is nothing our site could do to fix the issue, so that's why we encourage all users to manually check they don't own DLCs/packages before entering giveaways for them. If you happen to win a DLC/package, it would be a good idea to check your potential ownership of it before revealing the key, so keep this in mind for the future.