Let me modify it a bit,
Duel can be done in different ways like 2 vs 2 or 3 vs 3 or 4 vs 4. We can choose it.
Weapons will be same like before, no need to make it a choice since most of the people like this manner.
I don't know what would be the command, but for example /teamduel (your friends' ids) (enemies' ids)