By Default, when you add a product to your cart in Opencart 3X or hit Buy Now, normally if you go back to that product and add again, it will update your cart's quantity. Is there anyway I can provide an error that you have an existing product in your cart? and not allow you to continue? - This is for a special project that only offers one product per customer per order - However they are allowed to reorder, but not add or checkout more than one at any given instance. Like Supreme®
