Key is activated

Maybe duplicate key ?

Where did you get the key from?

The product code you've entered has already been activated by a different Steam account. This code cannot be used again. Please contact the retailer or online seller where the code was purchased for assistance.
Enter your product code to activate your product on Steam

Product Code

I'ts key

So you won a giveaway on Steamgifts and the key provided by the giveaway creator was already used?

Edit: Assuming that is the case.
Contact the giveaway creator and ask him/her to provide a working key. You can contact him/her by posting in the giveaway for example.
If he/she is not willing to provide a working key, you can either agree for the giveaway to be deleted or mark as "not received" after 7 days.