Tests. AFAIC, this isn't something that should be long living. If it is only duplicated in a couple of places and remains unchanged for years, that's probably

Tests. AFAIC, this isn't something that should be long living. If it is only duplicated in a couple of places and remains unchanged for years, that's probably fine too, because ya... no one is touching it. If one place does need to change and tests still pass that should mean that the other one didn't need to change and you've reaped the benefit from not prematurely abstracting. There are a lot of ways it could play out, though. Often the duplication is very local and obvious. I think a lot of people take "duplication is cheaper than the wrong abstraction" WAY more seriously than its intended. It's actionable way of saying "don't abstract early" as the counter that is usually: "But then I'll have duplication and DRY is the law." Like EVERY piece of programming advice, though, it's not universal.