[wizofaus]

But the divergence is more often than not unintended and causes inconsistent/ unexpected behaviour when a change is made in one copy and not the other, which is why it ends up using up more time in the end. And how is extracting a few lines of code into a function and calling that more complexity than having two identical copies of it?

[IggleSniggle]

When the divergence comes from subtle differences in requirements, then those subtle differences in requirements now need to baked into your single function (or, really, broken out from the function; but they have to be recognized as different to begin with). Now, the next time you need to address a new feature along one pathway, you must also be certain that you are not subtly breaking some completely unrelated feature requirement.