expressはエラーを報告します。エラー [ERR_HTTP_HEADERS_SENT]: ヘッダがクライアントに送信された後、ヘッダを設定することができません。

// Handler function for registered users
exports.regUser = (req, res) => {
    // 1. Determine if the username and password are empty
    // 1.1. Accepting form data
    const userinfo = req.body
    // 1.2. Determining whether data is legal
    if (!userinfo.username || !userinfo.password) {
        return res.send({
            status: 1,
            message: 'Username or password cannot be empty!
        })
    }
    // 2. Check if the username is occupied
    // 2.1. Import data manipulation module
    const db = require('. /db/index')
    // 2.2. Define the sql statement.
    const sql = 'select * from ev_users where username = ?'
    // 2.3. Execute the sql statement and determine if the username is occupied based on the result
    db.query(sql, userinfo.username, (err, results) => {
        // 2.4. Failed to execute sql statement
        if (err) return res.send({ status: 1, message: err.message })
        // 2.5. Username is occupied
        if (results.length === 1) return res.send({ status: 1, message: 'Username is occupied' })
        console.log('1');
    })
    I did not comment out the following res.send('reguser ok') used for testing when verifying if the username was taken, which caused res.send() to be called repeatedly, resulting in the error: Cannot set headers after they are sent to the client.
    //res.send('reguser ok')
}