Whether it is pistols at dawn, swords at noon, or knives under the light of the full moon, the duel can be one of the more intense dramatic situations to come up in a story. Even better, duels can serve as the beginning, middle, or end of a story. They can be the event that launches off an entire plot, spanning sessions of game play and culminating in things as large as wars. They can be the middle ground, a first meeting between the protagonist and antagonist, showing how much the gap has closed, and how far there still is to cross. Finally, they can be the end point of a story, when all debts are paid, and someone either gets the vengeance they've always wanted, or dies trying.
Now, if you're looking for duels then the system that you want to talk about is Legend of the Five Rings. Not that you need to be running things in L5R for duels to happen, but so much of the intrigue in the setting revolves around duels that the system actually has them. So much so, that the way you do your 'quick' design for your adventure is based on key terms from the dueling system. For those not familiar with the system, Challenge, Focus, Strike.
The first part of any duel is the challenge. Now, this doesn't necessarily mean when one person actually challenges the other, but can also include everything that sets up the duel. Now, the gauntlet can be thrown down in a number of ways, and how you throw it at your player should depend on them and the character that they are playing. The trick here though is to keep in mind that whoever is throwing the challenge, you need to be prepared to make them a big deal for the player, and thus possibly for the game. This may even involve giving the NPC a bit of plot protection, if only so you can live up tot he duel. Now, how much depends on you, the players, and the game, but I mention it because you may want to keep it in mind.
So, how do you get the challenge down? Well, it is simple. Hurt the character. Draw first blood, and make sure they know who did it. This can be as easily as a scar on the face to a duelist, stealing a mage's spell book, sacking a lord's town, or almost anything else you can think of that damages something the character has gained for themselves, and the player has shown that they like having. Be aware, that in doing this you may want to talk to the player about it as well and let them know that it is for a plot, and not just because you are trying to hurt them. Course, the better you know the player, the later you can let him know that there is going to be some pain coming up for them.
On the other hand, the player can be the one to throw the gauntlet. This can be a very fun way to bring back a minor NPC, or someone the players just casually destroyed/hurt without noticing them. At this point, it is the player that has thrown the challenge and the NPC is coming back for them. This usually will lead into the above paragraph, after all, a Player has little reason to accept a challenge...but when they give it, well, then you have them fired up.
Either way you do it, the culmination you want involves the PC coming after the NPC. You want to set this up so that they get one on one time for this meeting. After all, it's not a duel if it isn't one on one now is it? How you do this is up to you though, but some of the most fun you can have is getting the player to call their enemy out in front of a crowd. In a court setting, the town square, or even in the middle of a battlefield. Talk it out with the player before hand a bit, right when you are coming up to the point it could happen. Let them know that they can call the person out, then give them the choice if they want to or not.
The focus is the meat of the fight, it is the resonance and the drama that comes up in the story. In a lot of ways it is where the emotion and tension that make a duel so spectacular comes from. Two people, staring at each other and just waiting for the other to twitch. Waiting, watching, trying to hide any sign of emotion, trying to keep themselves in control so that they can be ready in that one moment.
For the purposes of the dramatic situation, the build up for the duel comes in what happens before the duel. Now, we already talked about hurting your player for the challenge, but what comes after that is just as important. If yo do the challenge right, then your player will probably be pursuing the NPC they're to have the duel with. What you need to do then is give them a series of close hits and misses. Times when they just miss the NPC, times when they meet but the NPC gets away. Times when the NPC hits them again, or they get to hit the NPC back. These moments, the near misses and chance encounters, will give a bigger meaning, and a bigger feeling of fulfillment when the player finally gets to have their final show down.
You can really play with this if the confrontations that happened before involved confrontations in the PCs specialty. Sword fighters that had previous duels, mages that had previous magic encounters. Reference those fights as you go through duel itself. "She twirls and stabs, an exact replica of the move that killed your wife", "He the drawing of an arcane sigil he sends bolts of eldritch energy at you, the same spell he defeated you with last time you met", "With a cocky smirk reminding you of the time he baited you into a check-mate during the chess tournament, he produces a knife". You can use this to help pull the player into the right frame of mind, get a bit more in character as you remind them of the times they tried and failed, or were hurt by this person.
Without this, or at least without attention to this, the pay off when you finally get to the duel just might not be there. You end up with, to take a shot at Star Wars, Anakin vs. Count Dooku in Attack of the Clones, as opposed to Luke versus Vader. You just have no emotion in the fight.
Strike is the pay off of the duel. The tension has built, the players are in position, and the only thing left is for everything to come to an end. Honestly, this is the easiest part of the whole thing. If you've played everything else up fine, then this is simply execution. Letting mechanics take over, and going with the flow of things. Have fun with it, and remember to amp up the tension, and to let it crash when it needs to crash.