You are able to fight other players with kits that are made by yourself! Simply run the command /duel Username to fight against your opponent using that kit! It will be on the first item in the GUI, it will start off as a premade kit however you can edit it by running the command /customkit and have tons of options to choose from!
All the items that are named "Air" can be swapped out with actual items, simply click on it and see what you can switch it with.
You may also notice at the top of the GUI, there is 9 items. The first four is relating to your armor that however the last 5 relate to the game settings. The stone sword repesents the name of the kit whilst the others repesent on what is allowed in the game that the kit will take place on.