Are you aware that you should have declined the game and ask the giveaway creator to re-roll the winner?

But this could have been avoided in many ways my friend. Make sure that your settings are right: https://www.steamgifts.com/account/settings/giveaways That means that Enable hiding GAs for games you already own. Furthermore do not forget to sync your account manually every time there is a change (game, group) on your Steam.

Also there is no such thing as "too late when I realized". You should have asked for a re-roll as Oppenhaimer says. To mark the gift as received and then regift it quickly in a public GA, hoping no one will notice, is against the rules. Have you read them?