What about when an ajax POST adds an item to a shopping cart? There is no unique identifier in the line item count, and thus no way to determine "account alrea

What about when an ajax POST adds an item to a shopping cart? There is no unique identifier in the line item count, and thus no way to determine "account already exists". Worst case scenario, user gets extra item(s) in their cart, doesn't really look at the totals, and orders, pays for, and receives them.