When your honor has been slighted or you just feel like a friendly spar, or anything in between, duels are here to resolve things.
A declaration that you wish to duel someone can be sent to anyone in the vicinity. This includes a number of options, such as:
A note on "Weapons Only": This only really matters in a few situations. First Ones are always able to take full advantage of their armor or lack thereof, so the defense value armor provides is always the same regardless of what is worn or how heavy it is. Equipment can make this fluctuate quite a bit, as can mounts though. If you're unsure, or don't trust who you're challenging, force this to weapons only for the fight to have the least variability to it. On the flip side of this, in duels, if a First One is not using armor or equipment to provide a defense value, they will focus exclusively on their weapon skill to provide that. Which probably makes duels a lot more interesting.
Similar to how batallas work, duels resolve in stages where both sides always attack and always get a result, meaning a duel to the death in Might & Fealty can result in both participants dying of their wounds. There's also always a chance that a duel will be more severe than intended, particularly with uneven match-ups in skills. Finally, there is an exception to both sides attacking and getting a result for every round. If one side is using a ranged weapon and the other isn't, the ranged side will get a free attack of opportunity.
A duel will always end one of three ways, the challenger wins, the challenged wins, or a tie takes place when both members of the duel suffer a sufficient amount of damage to trip the extent limit during the same match. On a related note, the game will not tell you who is more wounded when this happens--there are no doctors in Might & Fealty capable of understanding First Ones other than other First Ones, and they can't be trusted to not bend the story to suit their needs.