I cannot stress this enough: be careful who you give dueling scars to, and where those scars are located.
You might think you’re slashing them across the face to “teach them a lesson” or “put them in their place” or even “to give them something to remember you by”, but in practice all you’re doing is giving them a backstory, raising their sex appeal, and setting them on the path of revenge.
If you want them to come back five, ten, fifteen years from now, then by all means, be my guest. Give them the sexiest scar you possibly can. But they will be more determined and more talented than you and they will kick your ass and probably kill you. Even if they’re not objectively “better at dueling” than you are, and you initially seem to have the upper hand, they have the blessing of plot behind them and that character arc is about to come crashing down on top of your head.
If you’re going to scar them, do it in some uncool and embarrassing place. The belly, maybe, if you’re careful. Across the nipple is pushing it. A slash across the ass isn’t particularly noble or sportsmanly of you but it doesn’t really increase your opponent’s cool factor unless you’re in a farcical comedy. Scar them somewhere if you must but do NOT help them accessorize their tragic backstory.