This is because the duel the way it is known of today does not at all operate the same way that the duel used to. Now the rules and regulations have been tossed out and the word is mostly used as a synonym for fight. However, these rules are quite important when it comes to dream interpretation.
A duel is taken to mean something different today than it used to just a few years ago. Now a duel is taken to mean just a simple fight or even worse, just a simple competition of some kind. In reality it was much more brutal than that. When someone was challenged to a duel, it was certain that either the person challenged or the challenger would not be going home the next day. While they seemed to have civilized rules and all kinds of restrictive things that seemed to keep order, this did not change the fact that it was a life or death competition that would result in the death of one person.
If you have a dream in which you are in the middle of a duel then this can have a lot of different meanings that have a strong bearing on your daily life. Most of this information has to do with the idea that you are probably in what seems like an ongoing duel with someone in your life. You might be having this dream because you are literally in an argument with someone and some of the themes from that argument are carrying over into your dreams since it is something that you are so concerned about. You are having a dream about a duel because you feel like you are in an ongoing argument in life.
If you have a dream in which you are dueling with someone then it means that you have some sort of internal conflict which you are trying to battle through and make sense of. It also depends on who the challenger was. If you are the challenger then it means that you are aggressively trying to put down your internal conflict but if the other person challenged you it means that your internal conflict is threatening to take you down with it.