It sounds like you have some items in your catalog with an alpha numeric value for the ID column. If this is the case, then make sure the OrderDetails.ID column is set to text. also on the confirm page, double click the Store Order Details server behavior, and change the ID column to text.
For the duplicate orders, add the following to the success page after the </html> tag:
<?php
// Initialize the session.
// If you are using session_name("something"), don't forget it now!
session_start();
// Unset all of the session variables.
$_SESSION = array();
// If it's desired to kill the session, also delete the session cookie.
// Note: This will destroy the session, and not just the session data!
if (isset($_COOKIE[session_name()])) {
setcookie(session_name(), '', time()-42000, '/');
}
// Finally, destroy the session.
session_destroy();
?>