Humans will do this as well, especially inexperienced junior SWEs. Adding a new boolean parameter and some if statements here and there. After a while a seemingly simple function takes four boolean parameters that each control a little bit of what the function does.

The benefit is that a human who is a junior might need at least a few weeks to months of guidance to have a good taste of when to duplicate and when to DRY. An LLM likely already has good judgment, and your prompt merely needs to be activate this judgment.