There is an interesting post by Bhikkhu Body on this topic which paul1 shared with me recently: Dhamma and Non-duality
It is a thoughtful essay, but a bit provocative. Another helpful post by Akaliko is here: Duality or something else - #3 by Akaliko
Much of the debate around duality/non-duality depends on how we define it and what we are discussing. One could argue that if referring strictly to the concept of self, since existence in its true form is non-self, there is no duality. On the other hand, I think most would agree that samsara and nirvana are different, and this could be viewed as a duality.