I need help on how activation keys work

[Moop]

Look, I want to buy an activation key for a pack of a game with the DLC included but I don't know if it will cause a problem because I already have purchased the game, the game is Fallout New Vegas if you're wondering. (Thanks in advance)

[cSg|mc-Hotsauce]

If the key gives you anything you don't already have say, a single DLC you don't have, the key gets used and you get what you didn't already own.

[Ogami]

As long as the key you want to activate has content that you are missing it will work.
But it will only add the stuff you are missing.
So lets say in this case you own the base game "Fallout New Vegas".
Then you try to activate a key for "Fallout New Vegas - GOTY Editon" which also includes all the DLC for the game.
The activation will work and it will add all the DLC you are missing to your existing copy of Fallout New Vegas.
It will NOT give you a extra copy of the main game since you already own that.
That second main game from the GOTY edition will go "poof".
But if you just want all the DLC this is a good way to add them to your game.