Just like there is a very early atomic theory and theories of the infinite developed in Ancient Greece, is there any form of observable/measurable prelude or predecessor to Dualism in our current modern-day understanding possibly found in Greek Philosophy/Philosophical writing?
I thought about the Aristotelian "Healthy Mind in a healthy Body", but I lack any knowledge of sources to state that with any accuracy. In any case, there is my question.
Inheritance of philosophical traditions is always a thorny issue when discussing history. Whether one generation 'owes' something to antecedents, or if the philosophy is self-invented is often a matter less of evidence and more of rhetorical skill. You might have better results asking the question over at /r/philosophy .
However, most post-modernists will point to Plato as the philosopher of dualism par excellance. If we take Cartesian dualism as the duality between material and mental, and in particular the superiority of one over the other, then no better place to start than Plato's Allegory of the Cave. Simply put Plato's allegory suggests humans live since childhood chained in a cave, and all we 'see' of the world beyond our chains are shadows projected onto the walls of the cave by firelight, and sounds echoed off the walls. We do not see the 'real' thing, but we see the distorted shadows of it. What we think is 'real' is actually imagined, projected onto the shadows cast by essential, archetypal ideas.
It is the neo-Platonists of the second and third century CE whom we have to thank for the injection of dualism into Christian writings and thinking, and therefore passes down to Rene Descartes. Descartes was not describing universal human condition but describing the inheritance of Christianity which was borne in a late Greek world.
Rene Descartes took this in different directions, but here we have the split between the 'real' and the 'imagined', one being superior to the other.