How should a writer address criticism that a work is not original or that it has been done before?
Perhaps the problem lies in the definition of "original".
A writer is immediately shackled by the necessity of conveying concepts in terms that the prospective reader will relate to and appreciate. One cannot adequately explain a particular color or hue to the blind, or an unusual sound or pitch to the deaf. The concepts lie outside of such individuals' experience. The trick is not to create a story that is entirely outside the realm of human experience, but rather to engage the reader with a fresh account that causes the reader to set aside any predisposition to dismiss the work because "it's been done before".
As soon as the author creates the first character, there is a back story rife with "done before". Let me introduce you to "Jane Doe", created for this post. Insert an age--26. Jane has already grown to maturity. So there is a whole childhood and early adult life already in the can. Her whole life is indirect action.
What do we learn about Jane? Orphan? Siblings? Married? Race? Height and weight? Nationality? Educated? Virgin or sexually active? Whatever we pick for Jane, readers will have preconceptions and expectations.
The trick is to insert enough quirks or devices for Jane that the reader does not expect to make the work "original". In my last novel, I needed some demons. Demons are a dime a dozen. Some have horns, and scabrous skin, and talons or claws, and long pointed fangs. Others shape change into devious and insidious forms. I created "nano-demons", magic's answer to nano-technologies. Octodecillians of nano-demons! Sure, demons have been done before, but I had never seen nano-demons.
Anyone can make a burger, but there is still one place in town that stands out. It ain't original, but it's damn good. Get out your thesaurus and cook up an interesting recounting of human experience (or inhuman experience if such is your wont). Don't worry that "it's been done before". Worry that it hasn't been done this well.