Since around November, Humble Bundle has changed the method of redeeming purchases on Steam. Previously, a game/package key was given which could be redeemed via the Steam client. Recently, Humble changed to an alternate method that probably uses a new Steam API to interface with the account. Your Humble account needs to sign-in to your Steam account, which will ask for permissions to modify your Steam account (similar to apps using Facebook/Twitter for access). I have noticed that this transition hasn't been ideal - game duplication can occur. For example, I know for a fact that I have Killing Floor. Yet, on the latest bundle, when I clicked on 'redeem' for Killing Floor, it was added to my account - supposedly. This has happened for at least another four games. Once the game is 'redeemed', you can't go back and gift the key. Only when the API detects that you own the game already, can you then gift this as a HTML link to a friend. I'm assuming that there are various game and game packages on the Steam database, so this is why duplicates are happening. So, a warning - if you want to gift games to friends, manually double-check your inventory.