Thanks!!

In this case, you don't really need to worry about that disclaimer. In short, what it's telling you is "check whether you already have this before activating the key, because if you do, it's not our problem if you end up wasting the key".

The longer version is that, if you successfully activate the key in your account, Steam will not give you extra copies just because you already own the contents of the key (in part or in full). This isn't usually relevant with keys that activate just one base game or just one DLC [*], but it's particularly important with keys that includes base game and DLCs, because if you already own a part of the package, in practice the key will just give you the parts you don't own. In cases like that, it may activate even if you already own the entire thing, in which case you would waste the key and get nothing in return. In any event, you only actually have to care about it if you already own (some of) the contents of the key.

[*]: But sometimes it is, because of how Steam activations work; it's complicated. I could try to explain it, but then this explanation would be even longer and probably too confusing, so I decided against it.

Thank you very much for the giveaway!