How a code duplication detector caught its own author and why he fixed the code

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A developer's custom duplicate-code detector flagged a ten-line structural pattern shared across two checks he had written separately, believing them to be distinct. The tool works by stripping string literals and normalising accessor calls before comparison, revealing identical code shapes that differ only in variable names — invisible to standard text searches or manual review. Rather than adding an exception or raising the detection threshold, the developer refactored the repeated blocks into a single declaration read through one map. To verify the refactor was safe, he confirmed not only that duplication dropped to zero but that all build output remained byte-identical to the pre-refactor run. The experience highlighted that structural duplication is typically more dangerous than copy-paste, and that bending a rule the moment it catches you is precisely when keeping it matters most.

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