Variable is incremented twice in node.js http callback function

I was playing around with node.js and something strange happens when you run this code:

var http = require("http");
var i = 0;

function onRequest(request, response) {  
  response.writeHead(200, {"Content-Type": "text/plain"});
  response.write("You're number " + i++);
  response.end();
}

http.createServer(onRequest).listen(8888);

I would expect it to behave like a page views counter, but with each refresh of the browser tab i get the result of what seems to be i=i+2 instead of a simple increment. Could someone explain this behavior to me?