Duplicate Http requests when retrying in Elm 0.19

Goal: Ensure that retrying a failed HTTP request does not result in duplicate writes to the backend.

Constraints: Elm’s update function is pure, so emitting the same Msg repeatedly produces the same Cmd; the Http module provides no built‑in retry, backoff, or deduplication mechanisms, and the runtime does not track in‑flight requests.

Uncertainty: The core documentation does not prescribe a canonical way to store pending request identifiers or to generate idempotency keys, leaving the pattern to application code.

• What is the recommended way to store pending request identifiers in the Model to prevent duplicate Cmds?
• How can retry logic be implemented in update without causing duplicate Http Cmds when the same Msg is re‑emitted?
• Are there established patterns for generating idempotency keys that work with Elm’s Http.send?