Are you sure there's anything needing cleaning up?

If you're talking about duplicate code showing up in 50 places then your problem is not code duplication but incompetent developers not being able to maintain a project.

If instead you're talking about code with a passing resemblance showing up in 2 or 3 places then odds are you're actually looking at more maintainable code straight in the eye and you're not able to understand how that makes the project more maintainable.

The demanding / loud person can and should be ignored; as a developer, you are responsible for code quality and maintainability, not your / their manager.

For other code it's an absolute stink and i agree. But for data transforms... I've seen the alternative, a neatly abstracted in-house library of abstracted combinations of dataframe operations with different parameters and.. It's the most aesthetically pleasing unfathomable hell I've ever experienced.

So now, when munging dataframes, i will be much faster to reach for 'copy that function and modify it slightly' - maintenance headache, but at least the result is readable.

Are you sure it's code duplication?

I mean, read your own description: the new function does not need to support edge cases. Having to handle edge cases is a huge code smell, and a clear sign of premature generalization.

And you even admit the guy was more productive and added less bugs?

There is a reason why the mistakes caused by naive approaches to Don't Repeat Yourself (DRY) are corrected with Write Everything Twice (WET).

They just aren’t making changes to the shared function, so they don’t need to test existing functionality still works, just their single use case.