>> that should just remind the agent to refactor into a common function when appropriate.

This off-the-cuff statement buries so much complexity. Sure it catches new code the exactly implements existing code, but IME it is __way__ more common to need to slightly (or not so slightly) change existing code that can now be used by multiple consumers, and then delete the new "duplicate" code. That is not trivial and requires (1) judgement from your AI coder and (2) deep reviewer expertise from your human coder.