Duel mode specific rules
All types of duels are allowed (ranged, mounted, shield, heavy armor...) if both duelists agree
That rule isn't very clear to be honest. However, there is an old thread in the admin section that specifically says that weapon changing is allowed. There aren't any rules regarding picking up weapons, mostly because it doesn't happen a lot and it would be pretty much impossible to enforce any such rule (and it would make certain duels, like throwing lance duels, impossible). Personally I've always intepreted the above rule as: anything goes, if you disagree with someone's playstyle just don't agree to duel with that person (don't press f).