I have no idea what I'm doing differently because I haven't experienced this since Opus 4.5. Even with Sonnet 4.5, providing explicit instructions along the lines of "reuse code where sensible, then run static analysis tools at the end and delete unused code it flags" worked really well.

I always watch Opus work, and it is pretty good with "add code, re-read the module, realize some pre-existing code (either it wrote, or was already there) is no longer needed and delete it", even without my explicit prompts.