Expressjs / Nodejs Error: Can't set headers after they are sent.

user.js

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

// TODO: if you try existing email and exist. username, this will trigger an errors
// Error: Can't set headers after they are sent.
//  not sure why this  happens, it works sometimes


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, existingEmail) => {
    if (err) {
      return next(err);
    }
    if (existingEmail) {
      req.flash('errors', {
        msg: 'Account with that email address already exists.'
      });
      return res.redirect('/signup');
    } else {
      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');
        } else {  // if here... then unique email and unique username, so save it

          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');
              // res.redirect('/account');
            });
          });  // end user.save function
        } // end else if existing user
      });  // end User.findOne username
    }  // end else existing email
  }); // end User.findOne email



};  // end main func.