The key that I received is all ready used in another account.

Just wait a week and mark it with the "Not received" In the https://www.steamgifts.com/giveaways/won

Just leave a message for the creator and wait for a week or so. The creator of the giveaway will contact you with a new key, or saying he hasn't any other key, and asking you to accept deletion.

I find extremely basic politeness to agree for the deletion. I always, always do. Sometimes keys get revoked sometimes they are old keys.

What do you mean you can't contact the giveaway creator? Do you mean you pinged them on steam?

You can simply post in the giveaway they created for the game telling them the key didn't work. This is the easiest way to contact someone on here and probably what most people would reply to.

Thank you for answering so quickly and helpfully. I send a friend request on steam if he/she accept it I am going to ask.

One thing to bear in mind: don't feel like you're being "mean" by marking as not received, or even more that to be "nice" you should mark it as received - marking giveaways accurately is essential for how the site works.

Also you can mark it as not received and if the creator sees your messages after that happens (or sees they have a not received mark so actually looks in the giveaway for your message), and then fixes the issue (gives you a working key), you can change the feedback to received (no support ticket required either).

This is what I do if there is a problem, leave a message in the GA stating the problem then:

1. Try new key if one is provided.
2. Agree to deletion if no alternate key is available.
3. Mark as not received if no response after a week.