title: Only allow one instance when navigate with NavController
body: <p>I'm currently using Android Navigation Architecture in my project. It has a feature that can launch any fragment with a shortcut. Currently I'm using NavController to navigate to desired destination when clicking at a shortcut.
  But when I clicked a shortcuts with multiple times, every time a new instance of the fragment will be created.
  So, my question is, Is there any way to only accept one instance of a fragment when navigate to it with NavController? 
  I'm googling many times but found nothing. Thanks in advance.</p>