It's not that hard to make if you think straight.
The challenge command sets a variable for both players and display a PM for the second player asking if he wants to fight.
In the accept command it checks if the current player is being asked and if so teleport both players to the fighting spot.
In the kill signal check if killer and killed are challenging, if so display a message that the killer won and teleport him back.
Not that hard, huh?
Tip for everyone that "needs scripts". Try to think how the commands should function.
Make a textfile with what everything should do and check stuff you made.
Work clean