Since everyone can enter this game, even the ones who own it, you should check, if the winner doesn't have it in their account already.

It's true for any giveaway that leads to a package/bundle page instead of a specific product page... Steam doesn't bother to mark any form of collection as "owned", only each individual item's page, so SteamGifts has no way of knowing if somebody has the items in the package or not without a ton of extra programming that can be problematic. Peetfighter is just reminding you that since there's no "you already own this product" warning, sometimes people can/will enter that already own everything, and giving them the key will just be wasted so you should check the winner's account yourself to make sure they don't own the individual items before releasing the key. If they do, you can always do a reroll.

Due to the way the package is setup on Steam, it's looked at on SG as how DLC is, which means anyone can enter for this, even if they own it. Due to that as Peetfighter mentioned above, you'll want to check to see if the winner owns it already before delivering it.