(PHP、SQL)当使用Insert Into ..Select语句我得到同一行两次

INSERT INTO target_table (column1, column2, ...)
SELECT DISTINCT column1, column2, ...
FROM source_table;

<?php
$dsn = "mysql:host=localhost;dbname=database";
$username = "username";
$password = "password";

try {
    $pdo = new PDO($dsn, $username, $password);
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

    $sql = "INSERT INTO target_table (column1, column2, ...)
            SELECT DISTINCT column1, column2, ...
            FROM source_table";

    $pdo->exec($sql);
    echo "Data inserted successfully.";
} catch (PDOException $e) {
    echo "Error: " . $e->getMessage();
}
?>