I just wanted to know how you set up a duel in-game.

Well first find someone to duel with, join the server and set the agreed mod, afterwards (/duel start [amount of tc you're dueling]. Then join back in the game.

don't duel, unless you wanna lose your TC
you might be easier game than you think

tho, if you still want to do it
set mod you like
reset
set duel ^that command on post above
to stop duel /duel stop
(reset)
to enter /en to spec /sp