Basically, you should aim to minimise complexity in software design, but importantly, complexity is defined as "how difficult is it to make changes to it". "How difficult" is largely determined by the amount of cognitive load necessary to understand it.
If you're copying and pasting something, there probably isn't a good reason for that. (The best common reason I can think of is "the language / framework demands so much boilerplate to reuse this little bit of code that it's a net loss" — which is still a bad feeling.)
If you rewrite something without noticing that you're doing so, something has definitely gone wrong.
If a client's requirements change to the point where you can't accommodate them in the nicely refactored function (or to the point where doing so would create an abomination) — then you can make the separate, similar looking version.
I would embrace copying and pasting for functionality that I want to be identical in two places right now, but I’m not sure ought to be identical in the future.
Sure, we could take the Foo, Bar, and Baz tables that share 80-90% of common logic and have them inherit from a common, shared, abstract component. We've discussed it in the past. Maybe it's the better solution, maybe not. But it would mean that instead of maintaining 3 component files and 3 test file, which are very similar, and when we need to change something it is often a copy-paste job, instead we'd have to maintain 2 additional files for the shared component, and when that has to change, it would require more work as we then have to add more to the other 3 files.
Such setups can often cause a cascade of tests that need updated and PRs with dozens of files changed.
Also, there are many parts of our project where things could be done much better if we were making them from scratch. But, 6 years of changing requirements and new features and this is what we have - and at this point, I'm not sure that having a shared component would actually make things easier unless we rewrite a huge amount of the codebase, for which there is no business reason.
What made your team decide on that rule? Could your team decide to drop it since it hinders improving the design of your code?
If two countries happen to calculate some tax in the same way at a particular time, I'm still going to keep those functions separate, because the rules are made by two different parliaments idependently of each other.
Referring to the same function would simply be an incorrect abstraction. It would suggest that one tax calculation should change whenever the other changes.
If, on the other hand, both countries were referring to a common international standard then I would use a shared function to mirror the reference/dependency that they decided to put into their respective laws.