# `deslop/duplicate-type-definition`

Flag named type aliases/interfaces with the same structural shape duplicated across files; extract one shared type.

- **Category:** Architecture
- **Severity:** warn
- **Source:** deslop-js
- **Framework:** global
- **Enabled when:** react-doctor deadCode analysis enabled (default true); whole-project scan only — skipped in --diff/--staged modes
- **Documentation:** <https://github.com/millionco/deslop-js>

## Validation prompt

Use this to decide whether a fired diagnostic is real or a false positive.

Fires from detectDuplicateTypeDefinitions when two or more named declarations (TSTypeAliasDeclaration or TSInterfaceDeclaration, .d.ts/declaration files skipped) hash to the same structuralHash across at least two distinct files: the high-confidence reason reads 'N identically-named type definitions of the same shape across M files — extract a shared definition', and the medium-confidence variant reads 'N structurally-identical type definitions detected across M files under different names (NameA, NameB, ...) — confirm whether the rename is intentional', where the parenthesized list is the join of every distinct name in the cluster (so a three-name cluster renders three names, not two). Suppress when the matching types are coincidentally identical but model genuinely independent concepts (e.g. a Point in a geometry module and a Coordinate in a maps module, or two DTOs mirroring separate external APIs) — merging them into one shared definition forges a contract that would force unrelated code to change together and break the moment one side legitimately diverges; the different-names/medium variant explicitly asks you to confirm the rename is intentional and is the common false positive.

## Fix prompt

Use this once validation confirms the diagnostic is real.

When the duplicates are truly the same concept, delete all but one declaration, export it from a shared module (e.g. export type User = { id: string; name: string } in types.ts), and replace the others with import type { User }; for the different-names case, pick the clearer name and re-point every reference to it. Only do this when the types must stay in lockstep — if they describe separate concerns, leave them duplicated rather than coupling unrelated code to one shape. See https://github.com/millionco/deslop-js