The spectator mode is already (practically) implemented, and used to be available for a temporary period for players before it was disabled. The potential for abuse using 3rd party voice chats is too big. However, enabling it for duel specifically, since that is something that can appeal to many players and not harm at all (since you can see your own damage to the enemy and the dead person can already see both people's hp) is something that should IMO be enabled. A lot of players have to rejoin every round to spectate if they wish to.
