I gotta be honest, if a buyer doesn't read the invoice to see what it is they're buying they're really asking to get what they're pay for.
On the other side, maybe LL could implement a check to see if items are already in the cart and prompting if so.
I've been trying to debate against this solution but I don't see a good reason against it.
My debate has been, The buy now feature is very useful for when I want to buy something very quickly. If I'm adding something to my cart as a gift for someone else, I probably don't also want to buy it for myself, but if I did want to buy it for myself, using the Buy Now feature doesn't complete my purchase any quicker because I still have to go to my cart to pay for the gift item. Adding the item to my cart would actually be faster because I would only need 1 checkout.