Single-flight controls coordinate concurrent work on an entity. Idempotency keys let an execution path recognize repeated requests. Both reduce known duplicate attempts when they are applied consistently and within their defined scope.
An external system introduces another boundary. If it accepts a write but the response is lost, retrying may repeat the effect unless that system also recognizes the request. The platform cannot recover certainty merely by recording its own attempt.
Design for reconciliation
A connector needs to declare which outcomes it can verify, which keys the downstream service honors, and how a reviewer can resolve a pending or ambiguous result. Work performed outside the controlled path requires separate accounting.
The Fibric reference architecture therefore treats coordination, duplicate suppression, and outcome verification as related but distinct concerns. This note describes bounded controls; it does not establish exactly-once physical effects or universal delivery guarantees.