"Can't set headers after they are sent" and empty post request body

I've been following a tutorial for a login/register authentication application. I keep getting the error "Can't set headers after they are sent" whenever I send POST requests. I tried to console.log the request body and it were empty. I don't know where I did wrong. Please help.

users.js

var express = require('express');
var router = express.Router();
var User = require('../models/User')

/* GET users listing. */
router.get('/', function(req, res, next) {
  res.send('respond with a resource');
});

router.get('/register', function(req, res, next) {
  res.render('register', {title:'Register'});
});

router.get('/login', function(req, res, next) {
  res.render('login', {title: 'Login'})
});

router.post('/register', function(req, res, next) {
  var name = req.body.name;
  var email = req.body.email;
  var username = req.body.username;
  var password = req.body.password;
  var password2 = req.body.password2;

  if (req.files && req.files.profileimage) {
    console.log('Uploading file...');

    var profileimageOriginalName = req.files.profileimage.originalname;
    var profileimageName = req.files.profileimage.name;
    var profileimageMime = req.files.profileimage.mimetype;
    var profileimageExt = req.files.profileimage.extension;
    var profileimageSize = req.files.profileimage.size;
    var profileimagePath = req.files.profileimage.path;
  } else {
    var profileimageName = 'noimage.png'
  }

  req.checkBody('name', 'Name field is required').notEmpty();
  req.checkBody('email', 'Email field is required').notEmpty();
  req.checkBody('email', 'Email field is not valid').isEmail();
  req.checkBody('username', 'Username field is required').notEmpty();
  req.checkBody('password', 'Password field is required').notEmpty();
  req.checkBody('password2', 'Password does not match').equals(req.body.password);
  var errors = req.validationErrors()

  if (errors) {
    res.render('register', {
      errors: errors,
      name: name,
      email: email,
      username: username,
      password: password,
      password2: password2
    });
  } else {
    var newUser = new User({
      name: name,
      email: email,
      username: username,
      password: password,
      profileimage: profileimageName
     })
  }

  User.createUser(newUser, function(err, user) {
    if (err) throw err;
    console.log(user);
  })

  req.flash('sucess', 'You are now registered and may login');
  res.location('/');
  res.redirect('/');
})
module.exports = router;

app.js

var express = require('express');
var path = require('path');
var favicon = require('serve-favicon');
var logger = require('morgan');
var cookieParser = require('cookie-parser');
var session = require('express-session');
var passport = require('passport');
var LocalStrategry = require('passport-local').strategy;
var bodyParser = require('body-parser');
var expressValidator = require('express-validator');
var flash = require('connect-flash');
var multer = require('multer')
var mongo = require('mongodb');
var mongoose = require('mongoose');
var db = mongoose.connection

var routes = require('./routes/index');
var users = require('./routes/users');

var app = express();

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');

// handle file upload
var upload = multer({dest: './uploads'});

// uncomment after placing your favicon in /public
//app.use(favicon(path.join(__dirname, 'public', 'favicon.ico')));
app.use(logger('dev'));
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));


app.use(session({
  secret: 'secret',
  saveUninitialized: true,
  resave: true
}))

app.use(passport.initialize());
app.use(passport.session());

app.use(expressValidator({
  errorFormatter: function(param, msg, value) {
      var namespace = param.split('.')
      , root    = namespace.shift()
      , formParam = root;

    while(namespace.length) {
      formParam += '[' + namespace.shift() + ']';
    }
    return {
      param : formParam,
      msg   : msg,
      value : value
    };
  }
}));


app.use(flash())
app.use(function (req, res, next) {
  res.locals.messages = require('express-messages')(req, res);
  next();
});

app.use('/', routes);
app.use('/users', users);

// catch 404 and forward to error handler
app.use(function(req, res, next) {
  var err = new Error('Not Found');
  err.status = 404;
  next(err);
});

// error handlers

// development error handler
// will print stacktrace
if (app.get('env') === 'development') {
  app.use(function(err, req, res, next) {
    res.status(err.status || 500);
    res.render('error', {
      message: err.message,
      error: err
    });
  });
}

// production error handler
// no stacktraces leaked to user
app.use(function(err, req, res, next) {
  res.status(err.status || 500);
  res.render('error', {
    message: err.message,
    error: {}
  });
});


module.exports = app;

UPDATE: If I remove the validator codes, the error will be gone, but the post request body still empty

UPDATW 2: Error log

POST /users/register 500 26.552 ms - 2629
Error: Can't set headers after they are sent.
    at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:346:11)
    at ServerResponse.header (F:\Projects\nodeauth\node_modules\express\lib\response.js:718:10)
    at ServerResponse.send (F:\Projects\nodeauth\node_modules\express\lib\response.js:163:12)
    at done (F:\Projects\nodeauth\node_modules\express\lib\response.js:957:10)

    at Object.exports.renderFile (F:\Projects\nodeauth\node_modules\jade\lib\index.js:374:12)
    at View.exports.__express [as engine] (F:\Projects\nodeauth\node_modules\jade\lib\index.js:417:11)
    at View.render (F:\Projects\nodeauth\node_modules\express\lib\view.js:126:8)
    at tryRender (F:\Projects\nodeauth\node_modules\express\lib\application.js:639:10)
    at EventEmitter.render (F:\Projects\nodeauth\node_modules\express\lib\application.js:591:3)
    at ServerResponse.render (F:\Projects\nodeauth\node_modules\express\lib\response.js:961:7)
    at F:\Projects\nodeauth\app.js:99:7
    at Layer.handle_error (F:\Projects\nodeauth\node_modules\express\lib\router\layer.js:71:5)
    at trim_prefix (F:\Projects\nodeauth\node_modules\express\lib\router\index.js:310:13)
    at F:\Projects\nodeauth\node_modules\express\lib\router\index.js:280:7
    at Function.process_params (F:\Projects\nodeauth\node_modules\express\lib\router\index.js:330:12)
    at next (F:\Projects\nodeauth\node_modules\express\lib\router\index.js:271:10)
    at Layer.handle_error (F:\Projects\nodeauth\node_modules\express\lib\router\layer.js:67:12)
    at trim_prefix (F:\Projects\nodeauth\node_modules\express\lib\router\index.js:310:13)
    at F:\Projects\nodeauth\node_modules\express\lib\router\index.js:280:7
    at Function.process_params (F:\Projects\nodeauth\node_modules\express\lib\router\index.js:330:12)
    at Immediate.next (F:\Projects\nodeauth\node_modules\express\lib\router\index.js:271:10)
    at Immediate.<anonymous> (F:\Projects\nodeauth\node_modules\express\lib\router\index.js:618:15)