You are not forced to accept this win if you already have the game elsewhere, you are always free to refuse a win. Just wanted to let you know that if you had the game already, then that is the possible reason as to why you were able to enter the giveaway before.
In this instance you did exactly as you should have, you checked your ownership of the game and didn't reveal the key, instead telling the creator to reroll. However it is up to the giveaway creator if they want to go through with that or not, if they don't then you can simply mark the giveaway as "Not Received" when you can since you don't have to take the key.