There's no good way to stop alts that doesn't also make account creation in general a tedious process, which scares off more real users than it stops people making alts.
Any system that doesn't go insanely overboard (we ask for your ID + phone number) would have enough loopholes to allow alt creation, so there's not much sense implementing more ways to ban people other than what we already have in place - which is more than just infractions and account bans.
Well duels are annoying with alts, you don't know their skill level so you have to basically go into it blind because no way will they play serious otherwise.
Don't duel white belts
If they had to make an alt then it means they're more likely to scam you anyway, and if you don't care about that then well, guessing their skill is just another test for your luck.