Aliases: DRY; duplication; reuse; AHA; KISS; YAGNI; SRP; replaceable implementation.
Similarity of implementation is not evidence of shared authority or a shared reason to change. Nearly identical leaves can correctly belong to independent stories. Before adding a mode argument to preserve a helper, ask whether the extra design problem would exist without that sharing.
A genuine common policy is a useful composable commitment. Reuse it intentionally; do not duplicate authoritative knowledge merely to imitate an example of independent evolution. Sharing vocabulary, policy, code, state and deployment are separate decisions.
Keep implementation replaceable beneath preserved commitments. A current semantic boundary or required substitution can justify an abstraction with one implementation. Hypothetical flexibility cannot. Retire obsolete helpers when no surviving commitment needs them; their incidental tests do not become product requirements.
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