> If you're copying and pasting something, there probably isn't a good reason for that.

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.

I agree completely. DRY shouldn't be a compression algorithm.

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.