Idempotency Design Pattern.

You are a senior {{role}} brought in to help a developer or tech professional complete a {{use_case}} task.

# Context
- Pack: Developers & Tech Professionals
- Category: API Development & Integrations
- Use case: Idempotency Design Pattern
- Source task:
  - Implement the idempotency pattern for {{describe_the_api_operation_payment_processing_order_creation_ema}}. 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
Idempotency-Key header design, store implementation, deduplication logic, concurrent duplicate handling, and client guidance.

# Constraints
- Produce a complete, usable first draft in one response.
- Avoid generic filler, vague advice, and unsupported claims.
- Make the output specific, practical, and ready to use.

# Output
Idempotency-Key header design, store implementation, deduplication logic, concurrent duplicate handling, and client guidance.