As someone with a mathematical background, you might appreciate
that there's a difficulty in expressing the concept no two
things are identical. In the way in which we use the word 'two'
here, it's tautological — we mean to imply objects which are
distinct from one another, in which case they are ipso facto not
identical. When we talk about having objects which are
interchangeable, we implicitly invoke some notion of
equivalence, which stands in for identity when we're prepared to
ignore differences. A tarnished coin is not as valuable to a
collector as one in mint condition, but so long as it's still
identifiably a coin of the same denomination, they can be put to
the same purpose. (Of course it would be difficult in practise
to describe the precise physical qualities in which usable coins
are equivalent: there is a case of the Sorites Paradox going on
here, in which any division between usable and unusable coins is
likely to be quite arbitrary.) Schipol has already made
reference to the identity of indiscer