Trap The Duplicate Logging On FMS, LoggOff Old One And Accept New Instance.

onAppStart intitialize an object to hold all connected clients, maybe called userList:
application.onAppStart = function(){
this.userList = {};
}

Then when someone logs in, they of course pass in their username. Simply put that client in userList by its username.
application.onConnect = function(newClient, username){
this.acceptConnection(newClient);
this.userList[username] = newClient;
}

Then you can easily modify that code to boot the old client if that username is already in use:

application.onConnect = function(newClient, username){
if(this.userList[username]){
this.disconnect(this.userList[username]);
}
this.acceptConnection(newClient);
this.userList[username] = newClient;
}

See if that works for ya.