Issue in saving registered user to MongoDB in Node JS - Express application

I am new to Node.js and trying to create a chat application program. For that I have created a Signup registration form with express framework. The data will be saved in MongoDB. Application uses passport middleware signup functionality. Issue is when submitting a new user for second time I am not able to see the data in mongoDB, instead i can see only the first data. I set the mongodb debug option to true, after submitting the form, user submitted data will be seen through console.

Please see the github code which i created: https://github.com/Deepesh316/jabarchat

Please see the mongodb user details getting saved data code: passport-local.js

passport.use('local.signup', new LocalStrategy({
    usernameField: 'email',
    passwordField: 'password',
    passReqToCallback: true
}, (req, email, password, done) => {
    User.findOne({'email': email}, (err, user) => {

        // Network or Internet connection error
        if(err) {
            return done(err);
        }

        if(user) {
            return done(null, false, req.flash('error', 'User with email already exist'));
        }

        // Creating new instance of user and save it to database

        const newUser = new User();
        newUser.username = req.body.username;
        newUser.email = req.body.email;
        newUser.password = newUser.encryptPassword(req.body.password);

        newUser.save((err) => {
            done(null, newUser);
        });
    });
}));

Below is the code snippet for Model:

const mongoose = require('mongoose');
const bcrypt = require('bcrypt-nodejs');

const userSchema = mongoose.Schema({
    username: { type: String, unique: true },
    fullname: { type: String, unique: true, default: ''},
    email: { type: String, unique: true },
    password: { type: String, default: ''},
    userImage: { type: String, default: 'default.png'},
    facebook: { type: String, default: ''},
    fbTokens: Array,
    google: { type: String, default: ''},
    googleTokens: Array
});

userSchema.methods.encryptPassword = function(password) {
    return bcrypt.hashSync(password, bcrypt.genSaltSync(10), null);
};

userSchema.methods.validUserPassword = function(password) {
    return bcrypt.compareSync(password, this.password);
};

module.exports = mongoose.model('User', userSchema);