# HARN-MAT-002 — match expression contains a duplicate arm
**Category:** Pattern match (MAT)
**Variant:** `Code::DuplicateMatchArm` (duplicate match arm)
## What it means
A `match` expression is incomplete, ambiguous, or otherwise invalid. Harn
enforces exhaustive matching to keep agent decision logic auditable.
Specifically: match expression contains a duplicate arm.
## How to fix
- Add arms covering the remaining variants, or use a wildcard `_` arm as the explicit fallback.
- Remove duplicate arms — each variant should appear at most once.
## Stability
This code is stable. Its identifier, category, and meaning will not change
without a deprecation cycle. Cross-language tooling and IDE integrations can
dispatch on it directly.