Why: A busy 409 used to enqueue a new id, so the retry created a second user message after the server had already stored the first.
Evidence: vitest run passed, 13 files and 69 tests, including the outbox retry case.
Impact: Data layer only. sendMessage and outbox flush reuse the queued message id as requestId. No screen changes.
Refs: Dev feature/pi-durable-borrow
Co-authored-by: Cursor <cursoragent@cursor.com>