Create a giveaway with a key

You can't giveaway beta keys and you can only give away Steam redeemable keys.

Also keys for these games are not allowed:

Dead Island
Call of Duty: Modern Warfare 3
Bulletstorm
Homefront Shotgun DLC
AI War
Supreme Commander / Forged Alliance
Portal 2
DiRT 3
Puppy Games Keys
Red Orchestra: Ostfront 41-45 (Region Locked)
Dark Messiah of Might & Magic Multiplayer
Eets (in certain amounts)
World of Goo
Puzzle Agent
Risen 2: Dark Waters - Air Temple DLC

But if it's a different game from these and it's not beta then you can create a giveaway of that game, and send the key to the winner to redeem on Steam.

You won't lose the key if you redeem it yourself, it will just say you already own it, but it won't give you a gift either, so there's no point. It's perfectly fine to give away a steam key on steamgifts, and just send him the key, as long as it is for the full game, and not a beta key.

Just give the key to the winner...

Perfect, thanks.

Read the FAQ for the exact and officially banned games list, if the game is not on this list you can just send the key to the winner :)

Firstly, please carefully read the FAQ. It might answer some of your questions.

Anyhow- since you already have the game trying to activate it on your account will inform you that you already own it and the key won't be used. If the key isn't for one of the exploited games(see FAQ for what an exploited key is) you have no problem of making a giveaway with it and just give it to the winner. :)