We should be able to restrict users from adding a product to their cart if they already have that item in their cart similar to the way Creative Market handles digital products in the cart. This would benefit both store owners and their customers when it doesn't make sense to purchase more than one of the same product (especially digital products). When someone clicks the "add to cart" button for a product with this condition, the button for that product should either be disabled or replaced with a "view cart" button until the product is removed from the cart.