Arnold Zuboff makes a case for this in "One Self: The Logic of Experience" (Inquiry, 1990) [0]. He calls it universalism: there has only ever been one subject of experience.
One of his thought experiments: imagine a meteorite is about to wipe out all life around you right now, instantly. If this exact experience - being you, here, thinking this - is instantiated anywhere else in the universe, it will continue there without any gaps.
Hindu mystics were already exploring and articulating these non-dual concepts in great detail thousands of years ago, in the Upanishads. As Robert Jastrow would say:
"For the scientist who has lived by his faith in the power of reason, the story ends like a bad dream. He has scaled the mountains of ignorance, he is about to conquer the highest peak; as he pulls himself over the final rock, he is greeted by a band of theologians who have been sitting there for centuries."