Double elimination seems fine. I doubt there will be enough people (given the size of the Warband community and the restrictions applied) to form pools which in turn will make brackets. If you do have enough, use this format:
Pools with an amount of people determined by the overall size of the players that signed up. Fat players need not apply. For an example, let's use four people for a pool. With four people you have a round-robin style system where each duelist is faced off against the other three in their pool. Overall scores are tallied like so:
Player 1 scored 7-3, 7-5, 7-2.
Player 1 will have an overall score of 21-10.
Player 2 scored 3-7, 5-7, 2-7.
Player 2 will have an overall score of 10-21.
Assuming player 1 and player 2 are in the opposite sides of the pool (where the top two pass on and the bottom two are eliminated), player 1 will make it out of pools and player 2 will be eliminated (while still having a fair amount of playtime for having signed up!) or be placed in the loser's bracket (where a chance to advance still awaits). This creates a bracket where only the best players will make it out of their pools, making for a high level of competition in the winner's bracket.
Once the brackets are formed, the winner's bracket progresses normally with duelist-against-duelist eliminating each other. Once knocked out, a duelist in the top bracket can be sent to the bottom bracket for a second chance at making it to the finals. If the population was large enough, you could even hold secondary pools where eliminated players from the first round of pools can fight for the extra bottom bracket slots.
Just a thought, probably already thought of/how it's done, but it'd be interesting~
Miaou~! :3
Banned
even though I'm not good at duels 
