Yeah - that's a great idea. /duel [playername], and /duel accept. You could even make it have three settings like /duel [playname] then either 'fair', 'opfair' or 'ownkit', with 'fair' equipping both players with a mystic kit and 'opfair' equipping both players with a sovereign kit: i.e that level armour, sword, axe and bow, with an arrow and 64 steak and also disabling mcmmo. ownkit is obviously without changing kit.
When the duel ended it would delete that kit and return what was in the players' inventory before the duel.
Being in a /duel would mean both players could only harm each other, and couldn't be harmed by other players until one player does /duel cancel (which can't happen during combat time), or a player dies. That would actually fix a lot of issues with two people trying to fight then getting killed by someone else. There would have to be a timer on /duel though so that it ran out after maybe ten minutes. It would also have to only happen within /warp pvp.
As a sidenote - it would also be great to make it so that armour/weapons didn't lose durability within /warp pvp. I've had to stop fights several times because my armour is about to break, and with my current armour it breaks before I die. I've lost half an event v3 kit from this actually.