I think the problem is in this session variable:
$_SESSION['WADA_Insert_Items']
When you insert a new product this session is created and it only gets updated if a new product is created. to fix this issue try clearing out this session on the insert page by adding the following lines of code to the top of your insert page:
<?php
if (!session_id()) session_start();
?>
<?php unset($_SESSION['WADA_Insert_Items']); ?>