I Got a Gift by Code but it Says it's Duplicate Code

Please contact the the giveaway creator first to tell him about your situation

You need to say about it to GA creater.
If you are receivs non-ativate key then it is "not received" option.

Post in the giveaway letting the creator know and/or add them on steam. You can only mark as not received once 7 days passed from the end of the giveaway. Please use the search function (left top corner, just under the "new discussion" button).

You don't read FAQ... The creator have 7 days for send you code/gift and for this time 'not received' option can't be choose. After this 7 days you will can choose this option.

What happened after that?
Will I get another code or will I lose the game that I won?

Thank you very much guys, I'll try to contact the giveaway creator, I will update more information after I successfully contact him / her.

Okay, turns out it's already on my library. Maybe it's some sort of bug or something.
The chronology was like this:

1. I tried to redeem it while I'm offline ( with my internet on, I just set my status to offline ) and the Steam result is "Server Error".
2. I turn my status to online, and when I tried to redeem it I still got the "Server Error".
3. I restarted my Steam, and then I tried to redeem it again, this time it says "Duplicate Code".
4. I posted the problem here & tried to contact the giveaway creator.
5. I tried it again, this time it shows "Product Already Owned".
6. I checked my library and... Yeah, I just owned it today.

I assume the server error showed up because I used the HourBoostr to boost some game up while I'm offline.
It tends to happen, not just now, from the first time I used it.
Sorry for making trouble for you guys, and thank you for everyone who's trying to help, also thank you to the giveaway creator, for making the giveaway. :D