Why my middleware is not checking if the email already exists?

I am authenticating people using passportJS. The problem I have recently realised is the fact that users are able to register for more than one account. I have created a middleware to check whether the email is already in use but somehow it is still passing the test.

var User = require('../models/users');

var authMethods = {};

authMethods.isInUse = function(req,res,next){
  User.findOne({"email" : req.body.email}, (err,user) => {
    if(user){
      req.flash('error',"This mail is already in use.");
      res.redirect('/register');
    }else {
      return next();
    }
  });
}

module.exports = authMethods;

In my authentication page I am calling the middleware inside the route to meet the condition.

router.post('/register',authMethods.isInUse ,multipart(),function(req, res) {
  var image = fs.readFileSync(req.files.image.path);
  var profilePic = {data : image, contentType : 'image/png'};
  var user = new User({
      username: req.body.username,
      email: req.body.email,
      password: req.body.password,
      occupation: req.body.occupation,
      phone: req.body.phone,
      profilePic : profilePic,
      firstName : req.body.firstName,
      lastName : req.body.lastName
    });

  user.save(function(err) {
    req.logIn(user, function(err) {
      req.flash("success", "Welcome to the site " + user.username);
      res.redirect('/flats');
    });
  });
});

I haven't been able to spot the faulty approach that's causing the problem.

via Ozan