The worst duplication: the same rule written three ways across layers

Franc0Fernand0 · x · 2026-09-17

Franc0Fernand0 on the sneakiest form of code duplication: the same rule expressed three ways — a permission check in the API, one in the service, one in the database. No diff or review catches it; each version looks correct alone. Then you change the rule in two places and production still runs the old one. Semantic duplication like this can't be caught by tools and requires architectural discipline to eliminate.

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