socket.emit() fires twice - socket.io - node.js

Question

I'm making a one-to-one chatroom.
Here is my basic algorithm:
accept user's input (nickname) by a form
fetch all the existing connections and rooms from server
check if this nickname already exists:
if exists, form submission return false ; if not exists, ......
Now I'm having a socket.emit() twice bug on step 2.
Here is my client side code:
$('#login').submit(function(e){
socket.emit('getRoomInfo');
socket.on('receiveRoomInfo',function(info){
//Here has some code to fetch person_room list from server.
//e.g, person_room[nickname]= roomNO
if(person_room[nickname]){
alert('User already exists!');
}else{
//show chat
//hide the form
//update person_room list
//do something
}
});
return false;
});
Here is my server side code:
socket.on('getRoomInfo',function(){
socket.emit('receiveRoomInfo', {'person_room': person_room, 'roomNO': roomNO});
});
Bug Description:
Suppose I open up one form (index.html), and input 'John Snow', submit the form. Everything works fine, form gets hidden, chat shows up. Waiting for
the other connection...
Then I open up another form (index.html), and input 'John Snow', submit the form. Window alerts 'User already exists!'. Form isn't submitted.
I erase 'John Snow' and input 'Arya Stark', submit the form. Window still alerts
'User already exists!', but this time, form gets hidden, chat shows
up.
What I've test:
Login form is only submitted once
On client side, socket.emit('getRoomInfo') is triggered twice under Bug Description 3, which means socket.on('receiveRoomInfo') is triggered twice as well. So what happens is, 'Arya Stark' gets pushed into the list by the first time, and in the second time, it turns out she already exists in the list, Hence window still alerts 'User already exists!'.
I've researched a lot of similar questions, but none of them is the same case as this one.
Thanks for help.

Answer 1

I can fix the code, but I'm not able to tell the reason behind this bug.
I discovered that under once form submission, socket.on('receiveRoomInfo') is triggered twice.
So I set a variable var userExists=false under form.submit(),
after detected if(person_room[nickname]), set userExists=true.
Under socket.on('receiveRoomInfo'), surround all the code implementation with if(!userExists), so that we can make sure socket.on('receiveRoomInfo') is only executed once under once form submission.
I will be waiting for few days for a better idea before I vote my own answer.