Error: Can't set headers after they are sent.
at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:356:11)

/**
 * GET /signup
 * Signup page.
 */
exports.getSignup = (req, res) => {
  if (req.user) {
    return res.redirect('/account');
  }
  res.render('account/signup', {
    title: 'Create Account'
  });
};

/**
 * POST /signup
 * Create a new local account.
 */

exports.postSignup = (req, res, next) => {
  req.assert('email', 'Email is not valid').isEmail();
  req.check('username', 'username must be 6-50 lower case characters, only letters and numbers, no spaces allowed.').isAlphanumeric().isLowercase().len(6);
  req.sanitize('email').normalizeEmail({
    gmail_remove_dots: false
  });

  const errors = req.validationErrors();

  if (errors) {
    req.flash('errors', errors);
    return res.redirect('/signup');
  }

  const user = new User({
    email: req.body.email,
    username: req.body.username,
    // password: req.body.password
  });

  User.findOne({
    email: req.body.email
  }, (err, existingUser) => {
    if (err) {
      return next(err);
    }
    if (existingUser) {
      req.flash('errors', {
        msg: 'Account with that email address already exists.'
      });
      return res.redirect('/signup');
    }
  });

  User.findOne({
    username: req.body.username
  }, (err, existingUser) => {
    if (err) {
      return next(err);
    }
    if (existingUser) {
      req.flash('errors', {
        msg: 'Account with that username name already exists.'
      });
      return res.redirect('/signup');
    }

  });

  user.save((err) => {
    if (err) {
      return next(err);
    }
    req.logIn(user, (err) => {
      if (err) {
        return next(err);
      }
      // doesn't wokr either res.redirect('/account');
      return res.redirect('/account');
    });
  });
};