I'm experiencing a rare issue where existing customers completing checkout place the order successfully. However, the following strange behavior occurs:

 - The order confirmation email is not sent,
 - The order is assigned to the customer correctly, yet customer_id is set to 0, and
 - If the customer places the order again, it works correctly (order confirmation email sent and customer ID assigned correctly) **however**, the `increment_id` assigned is precicely the same as the first order and as a result there is a duplicate order increment id, with both orders active.

As I mentioned, the issue is quite rare, we receive over 20 to 30 orders a day, and it only seems to happen once a day, if that.

**Edit**: as per comment, it is useful to note that I am using an extension which allows alphanumeric order increment_ids, [Aschroder Set Custom Order Number][1], our orders follow a trend as follows: OR1000, OR1001, OR1002, OR1003, etc. We have been using this extension for over 10,000 overs without this issue appearing before.

I am also using [ASchroder.com SMTP Pro Email - Free and Easy Magento Emailing for SMTP, Gmail or Google Apps email][2] to integrate google mail support for Magento.

Is there a way that I can diagnose what is causing this behavior? All other orders placed seem functionally correct.


  [1]: http://www.magentocommerce.com/magento-connect/aschroder-set-custom-order-number.html
  [2]: http://www.magentocommerce.com/magento-connect/aschroder-com-smtp-pro-email-free-and-easy-magento-emailing-for-smtp-gmail-or-google-apps-email.html