It is not possible to only redeem the DLC, rather the whole game will be unlocked. If the redeemer (like in the case of the person asking the question) redeems the key, they will have the ultimate in their library (no matter, if they already owned parts of the package or not), There won't be a way to redeem the base game separately.

The reason they asked is: People who make a giveaway (like you in this case) are allowed to say, that they only want to give the game to people, for whom no part of it would go wasted, or only up to a maxium of x% of it.

So they want to know if it is fine with you, if they redeem despite having the base game already, or if you want to gift the game only to someone, who doesn't own any part of the package yet.

This common courtesy is based on this part of the rules: "Try to refrain from entering giveaways for packages if you already own the majority of the games contained within. The giveaway creator may request a new winner if they believe you own too many of the packaged games."