It will work. It will give you all the games in the bundle that you're missing. So if you have The Journey Down Chapter 1 and you activate the key then you will continue to have Chapter 1 on your account and get Chapter 2 as well.

But it's considered impolite to enter for a bundle that contains games you already own unless the creator of the giveaway says that it's alright to do so. Since the first chapter is widely-owned, that's why most of the Journey Down bundle giveaways have a comment asking if it's okay to enter just for the second chapter.