No need to delete it, the key will activate just fine, but the base game of this pack will be "lost" (as always on Steam, they will upgrade your packs but won't give an extra copy for already owned games)

So it's usually common/nice practice to ask for permission to join to the gifter, since (as per site rules) they can decide who can join:

Try to refrain from entering giveaways for packs if you already own the majority of the games contained within. The giveaway creator may request a new winner if you own a high percentage of the packaged games.