I supposed spells used in the duel would be up to the duelers' discretion? Wizards have many more options than the usual point and shoot way of muggle dueling. I imagine "throwing away your shot" would result in some sort of defensive spell, miss, or showy charm. As for offensive spells, I think it might depend on how much the two parties felt slighted from the situation. One person might want to be sure to instill a dire warning and cast a spell that has the potential to be fatal or bring about serious harm. Another person might use a common dueling jinx just to make a point or give the other party a scare. Wizard honor duels sound like they'd have much more potential to have more depth than their Muggle counterparts.

That being said, I suppose where, when, and what spells are being used might dictate if/how they could be punished. A duel that errs on the side of caution that's done in some remote location in the Hebrides probably wouldn't require consequences, as opposed to a duel seeking to serious maim another that takes places in Padmore Park. ?

Edit: I imagine wizard duels might be more a show of force than seeking to actually kill someone? I imagine if it ends up as the latter, a trial would have to be had and it determined if there was intent, or if it was an accident?