But if I fix all X bugs in code block A, then a code block containing B+A still contains X bugs.

That's one of the big problems with code duplication. "You fixed a bug? Great. Did you fix it everywhere?" It's better if there's only one place to fix it, because when you've fixed it, you've fixed all of it.

But as others have said, eliminating this is not the only good thing in programming. There are limits to how far you should go to prevent or eliminate duplication.