Client sends idempotency key

Server checks for duplicate requests

Submit button disabled during request

Button re-enabled in finally block

Database unique constraint on idempotency key

Duplicate handled gracefully

Non-duplicate errors not swallowed

5xx errors allow retry

Idempotency-Key header sent by client

Server checks idempotency key

Submit button disabled during request

Button re-enabled in finally block

Server-side duplicate prevention

5xx errors not cached

TTL on cached entries

Idempotency-Key header read

Cache lookup before processing

Response stored after processing

Lock key as processing

409 Conflict for concurrent duplicates

5xx errors not cached

TTL on cached entries

Cached status code preserved

Idempotency key accepted

UNIQUE constraint on idempotency key

Duplicate returns existing record

Non-duplicate errors re-thrown

Processing state tracked

409 or retry for concurrent duplicates

5xx errors not cached

schema.sql present with idempotency column