Triggers on the map from the editor will execute for all connected clients and the server. So if you have a ONCE trigger and there's two people playing you're gonna spawn three planes. One for your copy of the trigger, one for your friend's copy of the trigger and a third one for the server itself.
One way around this is to change your condition to this:
this && isServer
That way the trigger will only execute if it goes off but only for the server, not the players.
Feel free to post the mission here early to get some feedback and ideas for polish or more efficient ways of doing things maybe.