unique method

Implementation

void unique() {
  if (_head == null) return;
  _DoubleListNode<T>? current = _head;
  while (current != null && current.next != null) {
    if (current.value == current.next!.value) {
      _unlink(current.next!);
    } else {
      current = current.next;
    }
  }
}