Miért insertel duplán (PHP, MySQLi)?

A kód:

<?php

require($_SERVER["DOCUMENT_ROOT"] . "/config.php");

$conn = new mysqli(DB_HOST, DB_USER, DB_PASSWORD, DB_NAME);

$stmt = $conn->prepare("INSERT INTO test(first, last, city) VALUES (?, ?, ?)");

$stmt->bind_param("sss", $first, $last, $city);

$first = "1";

$last = "1";

$city = "1";

$stmt->execute();

$stmt->close();

$conn->close();

?>

A test táblában van:

id: INT AI PRIMARY KEY

first, last, city: varchar(65)

A kapcsolat adatai konstansként vannak beállítva a config.php-ban