Re: I already own Battlefield 3 and I got it via the Humble Bundle, what will happen to it?

The Battlefield 3 included in the Humble Origin Bundle is the base game, so you won't unlock any DLC by adding the code to your account. If you would like to gift your additional Battlefield 3 to a friend, simply send him the code. At the moment Origin does not include a gifting option and redeeming the code on your account would basically just destroy the code, since there is already a Battlefield 3 on your account.