Whatever Oda allows in a sword duel is pretty arbitrary, but obviously the fight has to be primarily with a sword and whatever you're using to amplify your swordsmanship as opposed to something like shooting lasers, lightning, magma, and black fire nukes.
It isn't a sword duel if your opponent switches from a sword to a flail to a mace mid fight, it has to be primarily with a sword.
This should be obvious because King vs Zoro makes a distinction between a sword duel and an all out fight where everything goes.