Marketpress: Authorize.net says shopping cart is generating invoice numbers incorrectly which allows

My client called to report a problem with an Authorize.net credit card transaction and the cart. A customer accidentally clicked the confirm button twice on their order and was charged by Authorize.net two times for the same order.

Usually this doesn't happen if the errant click takes place under a certain period of time (I think within 2 minutes) and Authorize.net won't charge the customer twice. When my client called Authorize.net she was told that yes, that is normally the case, but the shopping cart is "set up wrong" and generating a new invoice number each time the button is clicked so to them it looked like two separate charges.

I am not seeing anyone else reporting this bug, so I am not sure if it is indeed what happened. I do want to know if this is something we can prevent from happening in the future, however.

My client's busiest season is coming up very soon, so she will be getting slammed with orders for the holidays. I need to be sure that the cart is up to the task.

Thanks for your help,
Sarah