Your idea needs to pass the novelty test, that means it has to be significantly different from the previous patent. There is also something called a new use patent: you can have the same product but it is used in a new, novel way. For example, Melamine foam existed for decades and was largely used as sound proof insulation. BASF, the manufacturer, discovered that you can use it to clean your home and got a patent for the new use. They have made a fortune selling Melamine under the Magic Eraser brand.