If we add some log inside connection event handler, like

io.on('connection', function(){
  console.log('connection.');
});

It will output twice.

After a briefly investigation, I found socket.io cpp client sent a "connect" packet to the server after connection has been established. Server side seems to recognize it as a new connection. So "connection" event triggered twice.

Currently, I deleted

m_client->send(p);

from sio_socket.cpp (line 269). Then the second "connection" is gone. But I'm not sure if the "connect" packet is necessary or not.