Accidentally tried to activate a key for a game I already had. Now what? [duplicate]

If you already have that exact game / game & DLC bundle then Steam will refuse to activate it stating that you already own it.

You are then free to give it to someone else.

However if you do not own a portion of what the key contains it may get activated and just give you the bits you do not own. E.g. if its a game and DLC you may get the DLC and lose the extra copy of the game.

Quoted from the Steam Refunds page, here:

PURCHASES MADE OUTSIDE OF STEAM
Valve cannot provide refunds for purchases made outside of Steam (for example, CD keys or Steam wallet cards purchased from third parties).

You can try contacting Humble Bundle, though, as they seem to be able to revoke activated keys:

What happens after Humble Bundle revokes? Do they remove the game from the account? If you enable it in Steam they can't remove it, right?

Oh yes they can. A lot of people are scammed on trade forums, and keep trusting obvious scams, after which their games from Humble Bundle that they traded for TF2 keys are revoked. Maybe these scammers use Humble Bundle Support to tell "they activated game in an incorrect account" or something like this, or they just use fraudulent payments. Why would Humble Bundle Support remove the game and not make Steam revoke it from the account? What would be the point? People could just farm free games this way. [redacted for readability]