I almost got charged TWICE

I was at Fry’s Electronics to pick up a DVD that they had on sale. I bought a bottle of water from the vending machine. I carried the DVD with me up to the counter and I set the bottle up there close to me. I had already drank about a third of it. The cashier scanned my DVD and then scanned the water bottle that I had ALREADY PAID FOR. They sell the same water outside the machine. I think they should either not sell individual items on the floor that are also in the vending machines or put in vending machines that give receipts.