The system requires all products to be uniquely named. So if you need the same items in different categories. Ex. You have "Burger" in the specials category, "Burger" in the Lunch category and "Burger" in the Dinner category, these need to be uniquely named.
You can just add the category name to the end of the product name. "Burger - Specials", "Burger - Lunch". This will make the item unique and you can create it in different categories.