The same exported `interface` or `type` alias, with the same name and body, declared in two TypeScript files. Two sources of truth for one shape.
Each example shows the exact code shape the rule is looking for, then the smallest version that keeps the intent without hiding risk or adding noise.
// src/billing/types.ts
export interface Customer { id: string; email: string }
// src/email/send-receipt.ts
export interface Customer { id: string; email: string }Deterministic AST match across files: normalizes every exported interface and type alias in `.ts`/`.tsx` files and flags a declaration whose name and text match one already seen in another file.
// src/billing/types.ts
export interface Customer { id: string; email: string }
// src/email/send-receipt.ts
import type { Customer } from '../billing/types'Keep one exported type and import it where it is needed.
Agents declare types locally for the file they are working in instead of finding the existing one. The copies drift as soon as one is edited.
ai-slop/duplicate-type-declaration
ai-slop
Deterministic AST match across files: normalizes every exported interface and type alias in `.ts`/`.tsx` files and flags a declaration whose name and text match one already seen in another file.
Duplicates within the same file, types with the same name but a different shape, non-exported types, and non-production paths are not flagged.
Part of aislop's defensive-pattern detectors for TypeScript, scored in the maintainability tier because duplicate types create drift risk.