Duels in Gamemodes lack a spectate ability, and a new flag like
/join duel -spectate could work. Using a spectate flag in the command puts you in an already existing duel lobby as an invisible spectator. If no duel lobbies exist at the time of doing the command, it fails and gives off an error message along the lines of:
There are no spectatable duels going on as of now. Try again when there is one.
Also, you can make duels that don’t allow spectators with a flag like
/join duel -private.