If two things share literally every property, Leibniz said they must be one thing

Leibniz argued that no two distinct objects can be exactly alike in every respect: if you could list every single property of X and every single property of Y and the lists matched perfectly, X and Y wouldn't be two things at all, just one thing described twice. It sounds obviously true until you try to apply it to physics, where quantum particles of the same type seem to share every measurable property yet still count as separate particles — a case philosophers still argue over today.

— Gottfried Wilhelm Leibniz, Discourse on Metaphysics — 1686