Multiple identical orders received simultaneously, leaving negative stock

[omnicognate]

Using ZC 1.3.9h.

This is really odd. We have a product which had only one unit left in stock (products_quantity = 1). We just received 3 separate orders for this item from the same customer, one unit in each order. The orders are all listed in state "Processing" in the admin interface, and they are listed as all having been received in the space of 3 seconds: 17:17:36, 17:17:37, 17:17:38.

The stock level now reads -2, and "low stock" warning emails were sent to us with stock counts 0, -1 and -2. The item should have been listed as out of stock on the website when the stock level reached zero, and indeed is now showing as sold out.

We have, so far at least, only received a single actual payment, which came via paypal, which is enough to cover only one unit of the stock.

My strong suspicion here is that the customer actually only placed a single order, and that something has gone wrong with the paypal checkout process or within zen cart itself, that has resulted in the order being recorded 3 times.

Does anybody have any ideas? The site has been running for 3 years or so without seeing any such behaviour.