So updating my store I noticed that the cart behavior is kind of strange. First the overlay shows scrollbars with the cart information duplicated. Then when you checkout it seemed to duplicate the order so a quantity of one became a quantity of two, despite showing just one in the cart. So then I looked at the checkout button and noticed that the FCSID was being passed twice like - https://url.foxycart.com/checkout.php?fcsid=b9j8h0gvr0hel8tlq6n5kqu0n4&fcsid=b9j8h0gvr0hel8tlq6n5kqu0n4
The work around was to and quantity_max for the items, but that's not an ideal fix. I'm not sure if there's anything in the templates for changing how that button works.