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The response for any request made with an idempotency header is cached, and a subsequent request with the same idempotency key will return the cached response, including the initial response status code. The exceptions are requests that returns a 500-series status code (since they may not have been fully executed) and requests that are rate-limited and return a 429 status code. In these two cases, a repeat of the request with the same idempotency key will re-execute the request and not return a cached response. Our endpoints support idempotency as defined in the IETF draft specification. To send an idempotent request, add the header Idempotency-Key: {KEY} to your request, where {KEY} is an arbitrary unique value that you define. Add the same header and key value to each repeated idempotent request. Each idempotency key and any resulting cached responses persist for 7 days and then disappear. An example of a cURL request using an idempotent request header with an example key value of 259: 
curl -X POST https://api-sandbox.synctera.com/v0/customers \
  -H 'Idempotency-Key: 259'