We added code to the page that would prevent adding the same spot more than once. It depends on populating an array of previously selected spots. You could create a recordset to look up previous orders and pre-populate that array with the previously purchased values to prevent the same spot from being added by the same person again.
The checkout isn't working because it appears all of the content has been removed from the confirm_cart.php page. Usually when the continue button is pressed on the checkout form, that form is hidden and the confirm_cart page is displayed with a final checkout button. Since the confirm_cart page is blank the process is stalled at that point.
You can probably run the wizard again with new page names to get the necessary content for the confirm_cart page. I'm not sure how or when it was removed.
I would be able to help fix those problems in a premier support ticket if you have difficulty.