Quantum Mechanics and the Identity of Indiscernibles Essay by Allan Randall, relating Leibinz's criterion to Everett's possible worlds semantics of quantum mechanics. http://www.elea.org/Indiscernibles/
The Identity of Indiscernibles Entry by Peter Forrest in the Stanford Encyclopaedia of Philosophy. The principle is due to Leibniz, and states two entities are identical if they share the same properties. http://plato.stanford.edu/entries/identity-indiscernible/