A related problem: duplication is not equality. If two things happen to be the same right now, it doesn't mean they are intrinsically the same thing. If you hav

A related problem: duplication is not equality. If two things happen to be the same right now, it doesn't mean they are intrinsically the same thing. If you have multiple products selling for $59.99 they shouldn't share a function to generate the "duplicate" price. Abstractions needs to be driven by conceptual equivalence, not value equivalence, where duplication is a good hint for a potential candidate of abstraction, but not the complete answer alone.