The same name is exported from multiple files. Barrel re-exports collide silently and importers can't tell which one they got. The check runs in the engine's finalize pass — per-file run collects export names into the shared corpus, then finalize groups by name and emits one Issue per duplicate set with the canonical occurrence as the primary span and the rest as related: Vec<RelatedSpan>.

// src/users.ts
export function format(u: User) { /* ... */ }   // flagged

// src/posts.ts
export function format(p: Post) { /* ... */ }   // also flagged (related)

// fix: namespace, rename, or pick one canonical home
export function formatUser(u: User) { /* ... */ }
export function formatPost(p: Post) { /* ... */ }

Intentional cross-boundary mirrors

Some projects mirror a name across a boundary on purpose — a client/server contract pair, or a public re-export that deliberately shadows an internal name. exempt_boundary_pairs exempts exactly those pairings and nothing else. Each entry names one boundary; its sides are path globs separated by |:

# cofferdam.toml
[checks."Design.DuplicateExportName"]
exempt_boundary_pairs = ["client/**|server/**", "packages/public/**|packages/internal/**"]

A duplicate set is exempt only when every occurrence matches a distinct side of a single entry. So:

• client/schema.ts + server/schema.ts → exempt (one per side).
• client/a.ts + client/b.ts → still flagged; both are on the same side, which is an ordinary collision, not a mirror.
• client/schema.ts + server/schema.ts + utils/misc.ts → still flagged; the third file is outside the boundary, so the set is not a clean mirror.
• client/schema.ts + utils/misc.ts → still flagged; only the declared pairing is exempt, so the file stays under scrutiny everywhere else.

More than two sides are allowed (client/**|server/**|shared/**). Globs are matched against the project-root-relative path; an unanchored glob also matches at any depth, so client/** covers packages/app/client/schema.ts. Sides should be mutually exclusive — a path matching several sides is assigned to the first unclaimed one in declaration order.

Fixtures: examples/duplicate_export_boundary/.

If you'd rather not enumerate boundaries at all, demote the whole check:

# cofferdam.toml
[checks."Design.DuplicateExportName"]
severity = "low"   # demote to info-only if your project relies on barrel collisions