Why do I get this error : Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

Question

I am trying to create a simple registration using passsport-local and passport-jwt modules using the following code as my reister root/controller:

var passport = require('passport');
const User = require('../model/User');

const handleNewUser = async (req, res) => {
    const { email, password } = req.body;
    if (!email || !password) return res.status(400).json({ 'message': 'Username and password are required.' });

    // check for duplicate usernames in the db
    const duplicate = await User.findOne({ email: email }).exec();
    if (duplicate) return res.sendStatus(409); //Conflict 

    try {

        const result = User.register(new User({username : req.body.email}), req.body.password, (err , user) => {

            if (err) {
                res.statusCode(500);
                res.setHeader('Content-Type', 'application/json');
                res.json({ err: err });
            } else {
                console.log('success');
                passport.authenticate('local')(req, res, () => {
                    res.statusCode = 200;
                    res.setHeader('Content-Type', 'application/json');
                    res.json({ success: true, status: 'Registration Successful' });
                });
            }
        });


        console.log(result);
        res.status(201).json({ 'success': `New user ${user} created!` });
    } catch (err) {
        res.status(500).json({ 'message': err.message });
    }
}

module.exports = { handleNewUser };

And this is the error I do get:

POST /register
undefined
node:internal/errors:465
    ErrorCaptureStackTrace(err);
    ^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client

And it happens here:

res.setHeader('Content-Type', 'application/json');

I don't know why do I get this error message and how can I fix it?