Name the duplicated concept. Identify the thing that should have one owner: environment, pricing rule, geometry source, state machine, data contract, renderer phase, API shape, UI state, or test oracle.
Find every current owner and consumer. Treat wrappers, aliases, pass-local constants, copied structs, and "temporary" branches as sediment until proven otherwise.
Promote the concept to its natural home. Pick the module that would own it from scratch, then make old call sites consume that owner directly.
Delete or collapse the stale path in the same pass when feasible. If a bridge must remain, make it tiny, named as compatibility, and give it a removal condition.
Verify behavior through consumers, not just the new module. A clean refactor is only proven when the surfaces that used to diverge now report or exercise the same source of truth.

Rules

Do not over-weigh the sunk cost of the existing architecture. "It already exists and works" is not an argument for keeping a shape — coding agents make large architecture switches cheap, so size a refactor by the quality of the end state, not by the volume of code it replaces. When behavior must survive, pin it with tests at the consumer surface and swap the architecture underneath — though most of the time even the old seams shouldn't survive verbatim: a big refactor is the chance to redraw them into the shape the codebase would want today, not to faithfully rebuild the old interfaces on a new foundation.
Prefer one shared primitive over N adapters. An adapter is acceptable only at an external boundary or as a short-lived migration seam.
Do not preserve dev-only compatibility by default. Unshipped scaffolding should move to the clean contract immediately.
Make ownership visible in stats, tests, or debug output when divergence was the bug class.
Update the spec or handoff with the new invariant, not the mechanical file list.
If the refactor starts widening into unrelated behavior, slice it: land the shared contract first, then port consumers in reviewable passes.

refactor-clean