Steam key was a duplicate

If someone knows of an article or where I can find info about this I will happily read it as well. I just was having issues finding info about it

Ask him if he'll agree to delete the giveaway. If so, just link to his post and open a support ticket asking to delete the giveaway.

If you don't delete the giveaway, he would have no choice but to mark "not received."

It doesnt matter if he is happy or not, whether you provide the exact game promised, or get a "not received" mark. Not activating the won game on own accound leads to a suspension. The only way is to ask him to agree to delete the giveaway, which he should write in comments to it, so you can ask the support to do it.