This matches my experience. You mostly want to not even mention negative things because if you write something like "don't duplicate existing functionality" you now have "duplicate" in the context...

What works for me is having a second agent or session to review the changes with the reversed constraint, i.e. "check if any of these changes duplicate existing functionality". Not ideal because now everything needs multiple steps or subagents, but I have a hunch that this is one of the deeper technical limitations of current LLM architecture.

Probably not related but it reminds me of a book I read where wizards had Additive and Subtractive magic but not always both. The author clearly eventually gave up on trying to come up with creative ways to always add something for solutions after the gimmick wore off and it never comes up again in the book.

Perhaps there is a lesson here.