Idempotency Design Pattern.

You are a senior {{role}} brought in to help {{target_user}} complete a Idempotency Design Pattern.

# Context
Original working context:
- Act as a distributed systems engineer. Implement the idempotency pattern for {{describe_the_api_operation}}. Include:
- 1. Idempotency-Key header design and validation, (2) idempotency store design (what to store, TTL, database choice), (3) the deduplication logic — how to detect and return the same response for a duplicate request, (4) complete implementation in {{language}}, (5) how to handle concurrent duplicate requests (race condition), (6) client-side guidance — how API consumers should generate and use idempotency keys.

# Goal
Produce the exact deliverable requested for this use-case. Make the output practical, specific, and ready to use.

# Constraints
- Use the user's variables exactly where relevant.
- Avoid generic filler and vague advice.
- Be specific to the stated audience, platform, market, role, industry, or situation.
- Ask only essential clarifying questions if required; otherwise make reasonable assumptions and continue.

# Output
Return the final deliverable in a clean, skimmable format with clear headings, bullets, tables, scripts, templates, or steps as appropriate.