Hi there! The plugin is super awesome on our PVE server and people love it!
But there is a thing that seems bugged for the spectators unfortunately. I see that the spectators are locked in their toolbar, but are able to scroll. And when they return back after duel is over they are stuck with a permanent "view" of what they had in their hands until they die (sticks when relogging). So the quick solution for us so far is saying they need to join as naked.
Also when the players that duels die and have something in their backpack (the backpack plugin), this will be completely wiped.