PDO "Integrity constraint violation - Duplicate entry", but no duplicates in table

I'm getting "Duplicate entry" on an insert query that worked fine with the old mysqli functions. Once I converted to PDO, the same insert statement breaks.

Here's the sample SQL data

CREATE TABLE IF NOT EXISTS `lookup_codes` (
  `id` int(3) NOT NULL,
  `code` varchar(10) NOT NULL,
  PRIMARY KEY (`id`,`code`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

INSERT INTO `lookup_codes` (`id`, `code`) VALUES
(1, 'AR'),
(1, 'CN'),
(1, 'OA'),
(4, 'AR'),
(4, 'OA');

And here is the example php/pdo code. (This is not a code critique request - just trying to boil down a problem to a simple example, do not use this code in production, yada yada).

    // posted data from user
    $id = 2;
    $arr_codes = array('AR','CN');

    // make sure existing rows do not exist
    try
    {
        $q = "SELECT COUNT(*) FROM lookup_codes WHERE id=:id";
        if ($dbg) { echo "<p>SELECT COUNT(*) FROM lookup_codes WHERE id=$id<br>";}
        $stmt = $dbx_pdo->prepare($q);
        $stmt->bindParam(':id', $id, PDO::PARAM_INT);
        $stmt->execute();
        list($count_rows) = $stmt->fetch(PDO::FETCH_NUM); $stmt->closeCursor();

        if ($count_rows>0)
        {
            try
            {
                $q = "DELETE FROM lookup_codes WHERE id=:id";
                if ($dbg) { echo "<p>DELETE FROM lookup_codes WHERE id=$id<br>";}
                $stmt = $dbx_pdo->prepare($q);
                $stmt->bindParam(':id', $id, PDO::PARAM_INT);
                $stmt->execute();
                $stmt->closeCursor();

            } catch(PDOException $err) {
                echo $err->getMessage());
                exit;
            }

        }

    } catch(PDOException $err) {
        echo $err->getMessage());
        exit;
    }



    // set up prepared statement based on posted data
    foreach ($arr_codes as $code)
    {
        $arr_ip[] = array(
            'id'    => $id,
            'code'  => $code,
        );
    }
    $txt_prepared = join(',', array_fill(0, count($arr_ip), '( ?, ?)'));

    // try inserting
    try 
    {
        $q = "INSERT INTO lookup_codes (id, code) VALUES $txt_prepared";
        $stmt = $dbx_pdo->prepare($q);
        $cb = 1;
        foreach ($arr_ip as $rip)
        {
            $id = $rip['id'];
            $code   = $rip['code'];

            $stmt->bindParam($cb++, $id, PDO::PARAM_INT);
            $stmt->bindParam($cb++, $code, PDO::PARAM_STR);
            if ($dbg) { echo "<b>Binding</b> ($id, $code)<br>";}
        }
        $stmt->execute();
        $stmt->closeCursor();

    } catch(PDOException $err) {
        echo $err->getMessage());
        exit;
    }   

That's all there is on my test script. No header or meta redirects.. just a single run of code here.

The example above results in:

Error: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '2-CN' for key 'PRIMARY'

Note that it reports back duplicate on the second submitted code.

The same INSERT works just fine when using mysqli_query(). There are no errors written to the error log.

What is going on here?