We made the server behavior update instead of insert to cover the case where people go to paypal, notice something is missing or wrong and then click back to go back to the web site and make a correction and then check out.
This seemed to be a lot more common than someone going through the whole process and clicking back to return to the site rather than clicking finish and then processing a second transaction.
I'd still recommend express checkout to avoid these issues entirely, but if you want to force an insert every time you just have to clear the OrderID session variable on the page that stores the cart. It identifies whether to insert or update based on the stored order session variable. If that is cleared on top of the page it will always insert.
