List: A feature to have option if the list want to have one or more than one same items.

When we make a list sometimes we really don't want any duplicate items (title, people, or image) but sometimes we do. It would be very helpful if there is a notifications "already on the list" or "this is already on your list" when items already been added especially if the list have hundred items, including with question "add to list again? this is already in your list" then the option can be "add anyway" or "cancel". So we can have the option to have just one item or more than one item each list.