About multiple wins

Depends on the game. But generally no, don't give him, imo.

What it means that if he won several of one he is reselling or trading those keys, especially since he don't own the game.

If they've already been banned for it, you can't get a reroll (without their permission).

report him for multiple wins

Exüanding on MoonQueen3:
Request a new winner, give the old multiple win as a reason, link both old wins for support.
Support will either give you a new winner or tell you that your original winner has already served their suspension. In the latter case, send your key.

Thank you all for the help.
I suppose there was no reason not to send in a ticket in situations like this.

Sounds like a red flag, and against SG tos.