Can't set headers after they're sent

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Current behaviour

I am trying to make an app with Node.js, Express and Socket.io. I’m using EJS as the view engine. This is the relevant part of my code -

const express = require('express');
const app = express();
//copied from socket.io tutorial 
const http = require('http').Server(app);
const io = require('socket.io')(http);
app.set('view engine', 'ejs');
app.set('views', path.join(__dirname, '../templates'));
...
app.get('/', function (req, res) {
    logger.info('index');
    return res.render('index');
});
...
const port = process.env.PORT || localConfig.port;
http.listen(port);

Now if I navigate to localhost:3000, I get the error

Error: Can't set headers after they are sent.
at validateHeader (_http_outgoing.js:489:11)
at ServerResponse.setHeader (_http_outgoing.js:496:3)
at ServerResponse.header (/home/aniket/Documents/HumbleHelper/node_modules/express/lib/response.js:767:10)
at ServerResponse.contentType (/home/aniket/Documents/HumbleHelper/node_modules/express/lib/response.js:595:15)
at ServerResponse.send (/home/aniket/Documents/HumbleHelper/node_modules/express/lib/response.js:145:14)
at done (/home/aniket/Documents/HumbleHelper/node_modules/express/lib/response.js:1004:10)
at tryHandleCache (/home/aniket/Documents/HumbleHelper/node_modules/ejs/lib/ejs.js:228:10)
at View.exports.renderFile [as engine] (/home/aniket/Documents/HumbleHelper/node_modules/ejs/lib/ejs.js:437:10)
at View.render (/home/aniket/Documents/HumbleHelper/node_modules/express/lib/view.js:135:8)
at tryRender (/home/aniket/Documents/HumbleHelper/node_modules/express/lib/application.js:640:10)

From the log, I can see that ‘index’ has been called twice, followed by the error. However, if I remove this line - const io = require('socket.io')(http);

Steps to reproduce (if the current behaviour is a bug)

Navigate the browser to localhost:3000 and it fails. I have tried a res.send() instead of res.render() but that fails too.

Expected behaviour

The index page should be rendered correctly

Setup

• OS: Arch Linux
• browser: Google Chrome 59
• socket.io version: 2.0.4

Other information (e.g. stacktraces, related issues, suggestions how to fix)

I’m also using Passport, Helmet, Body-parser and express-session in conjunction with Socket.io

[heraldofsolace]

This happens if I share the same port between Socket.io and Express

[heraldofsolace]

Thanks @rwalle61 for your help! As far as I remember I was following the default Node JS template for IBM Bluemix (It was for a hackathon), and the lines were already there if memory serves! Anyway, I completed the hackathon by removing those lines. Now I see why this happened! Thanks for your help!