The duplicate() method belongs to the concrete value member of the class iterator. It generates a duplicate of the iterator which will iterate over the alike order of values. The duplicate iterators are said to be equal if they are put at the identical element.
def duplicate: (Iterator[A], Iterator[A])
It returns a pair of iterators.
(non-empty iterator, non-empty iterator)
Here, the stated iterator is non-empty so, two non-empty iterators are created.
(empty iterator, empty iterator)
Here, the stated iterator is empty so, two empty iterators are created.