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Code hash index
Two addresses with the same runtime bytecode are the same contract, whatever they are called and whichever chain they sit on. Hash the code and that equivalence becomes searchable — which is how an unverified contract can still show you its source, borrowed from a verified twin.
What a twin gives you
Recovered source
If any deployment of this bytecode was verified, its source describes all of them — byte-for-byte identical code cannot behave differently.
Deployment spread
The same code at many addresses is a factory pattern, a proxy implementation, or a fork. The spread tells you which.
Immutables excluded
Constructor arguments live outside runtime bytecode, so contracts differing only in their configuration still match.