I am having an issue with Paypal sending IPN messages to my Magento 1.6.2 store a short time (30 minutes or so) after an order has been completed. When the message is received, the order is marked as "Processing" again. The problem here is that these orders are automatically downloaded into our backoffice Sage 200 system every 15 minutes, and marked as "Exported" in Magento at that time, so that the export schedule doesn't download them twice, and therefore create a duplicate order in Sage 200 (it only downloads orders marked as "Processing").
It doesn't happen to all orders, so I'm not sure why it happens at all? Any pointers on how to debug, and more importantly, prevent this from happening, would be greatly appreciated!