在将用户存储到数据库之前，我想检查该用户是否已注册，但当我尝试发布相同的已注册电子邮件时，它会显示正确的错误消息，但服务器停止说：error[err\u http\u headers\u sent]：发送到客户端后无法设置头
这是我的密码：

registerUser: (req, res) => {
        let sql = 'SELECT * FROM registeredusers WHERE email = ?'
        pool.query(sql, [email], (err, results) => {
            if (results.length === 1) {
                return res.status(400).json({
                    message: 'Email already exist.'
                })
            }
        })
        const saltRounds = 10
        var fullname = req.body.fullname;
        var email = req.body.email;
        var userPassword = req.body.userPassword
        bcrypt.hash(userPassword, saltRounds, (err, hash) => {
            if (err) {
                return res.status(500).json({
                    message: 'Internal error. Retry..'
                })
            }
            userPassword = hash
            userService.createUser(fullname, email, userPassword, (err, results) => {
                if (err) {
                    res.status(500).json({
                        status: 'error',
                        error: err,
                        message: 'DataBase Connection Error'
                    });
                }
                return res.status(200).json({
                    status: 'success',
                    data: results
                });
            });
        })
    },