The no-cloning theorem in a principle of Quantum Physics was first proposed in 1970, and has since held up to both experimental testing and mathematical proofs. The theory, in a simple form, draws on the Heisenburg Uncertainty Principle to show that it is impossible to create an identical copy of an arbitrary quantum state – the very act of trying to measure one to recreate it would change it. In theory, this means that making a perfect copy of anything is impossible.
However, this does not stop quantum data – called qubits – from entangling. In the Dark Stary Universe, the story posits that one copy is what is called an Imperfect Clone of one another, with entangled qubits. When one is forced to collapse into a known quantum state, it forces a change in the other entangled qubit. This results in two copies of an Exalted that act slightly differently, and otherwise-illogical changes in which one is the “original” to occur.
For reasons that I am not smart enough to really understand, this theorem is closely linked to another, called the No-Deletion Theorem. If one is true, the other is, by force, true. This theory has some of its own quirks as relates to Exalted, but probably of the most importance to them is that it proves the idea of Closest Continuation for consciousness – The quantum state of a copied human brain cannot be destroyed, so it must continue to exist elsewhere, in the closest place that it could exist… which would always be the newly copied Exalted mind. As far as the Exalted are concerned, this would be positive proof that they are not merely copies of a dead person… they are that person, and changing which body they are in did not change their consciousness. You can learn more about these fascinating bits of physics here.