Duplicate call to 'open' using mongoose

Good, starting at node.js and mongodb I'm first with a problem that I do not know if it is such.

Having the server.js files as simple as

'use strict';
var http = require('http');
var express = require('express');
var bodyParser = require('body-parser');
var dbapi = require("./db/db.js");

var port = process.env.PORT || 1337;

http.createServer(function (req, res) {
    res.writeHead(200, { 'Content-Type': 'text/plain' });
    res.end('Hello World\n');

    dbapi.connection();
}).listen(port);

and db.js with

'use strict'
var mongoose = require('mongoose');

module.exports.connection = function () {
    mongoose.connect("mongodb://localhost/protocoloDb", { useMongoClient: true });

    var db = mongoose.connection;

    db.on('error', console.error.bind(console, 'connection error:'));

    db.once('open', function () {
        console.log("Now connected!");
    });
}

The result in the console is a double 'Connection established'. Is not 'eleven' supposed to be executed only once?

Thanks