为什么MySQL中的单个INSERT生成无限数量的插入？

This code

<?php

    $link = mysqli_connect("localhost", "...", "password", "...");

    if (mysqli_connect_error()) {

        die ("No database connection.");

    }

    $query = "INSERT INTO users (email, password)
                 VALUES ('hello@hello.com', 'mypassword')";
    if (mysqli_query($link, $query)) {
        echo "New record created successfully.";
    }
?>

displays the sentence "New record created successfully." only once correctly, but inserts the same row with a different id forever. I suspect that the problem is related to the AUTO_INCREMENT property of the id field, but I can't understand this behavior.