If code block A has X bugs and code block B=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.
You've got one aspect, and the other aspect is that code factored into a reused function F is now reviewed more too, ie. whenever you're reviewing the callers, you work through the functions they call as well to trace the behaviour. So you're also much more likely to find any bugs in F.
It's a double-whammy for bug squashing, which is why DRY is such an important principle. Certainly there are some misapplications, but its benefits hold up pretty well in a wide variety of scenarios.