DEUL :-
EARLY GAME = KILL
MID GAME = TAKE OUT IMPORTANT ENEMY OUT OF TEAM FIGHTS
LATE GAME = WEAK CHANNELED DISABLE.

beginning of the game, duel is a finishing move.

middle of the game, duel is a 'remove that guy and kill him' moev.

end of the game, duel is a weak channeled disable that might kill a guy if you did a good job early.

https://www.reddit.com/r/learndota2/comments/4sn3eu/how_to_play_lc_and_not_suck/