[usrusr]

Even with "copy on write" there would still be the problem of wether to copy before write or not: requirements for usage A change because someone recently thought hard about that part. Usage B is most likely not on anybody's mind right now andchanging that is quite some effort. It's a problem no matter wether you copy, DRY or something in between.

Maybe some of our processes, language and experience from the version control space (branch, merge, rebase, upstream...) could be ported over to the reuse vs duplication problem. But when you layer that with the conventional use of those concepts in version control, the result would be quite the multidimensional mindbender. With that as a hypothetical baseline, simply guessing a good compromise between abstraction and duplication and then dealing with the consequences suddenly does not seem that bad a fate.