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​
type
string
invalid_request_error
​
status
number
409

​
What happened

You attempted to create a customer with an external_id that already exists in your account. The external_id must be unique per account. 
{
  "error": {
    "type": "invalid_request_error",
    "code": "CUSTOMER_EXTERNAL_ID_CONFLICT",
    "message": "A customer with external_id \"user_8821\" already exists",
    "status": 409,
    "doc_url": "https://done.mintlify.app/errors/customer-external-id-conflict"
  }
}

​
How to fix

Either use a different external_id, or retrieve the existing customer with GET /v1/customers and use its id directly. 
curl https://api.done.app/v1/customers \
  -H "Authorization: Bearer <your_token>"

​
Common causes

Your system called POST /v1/customers twice with the same external_id. Check for idempotency issues in your provisioning flow and handle the 409 by looking up the existing customer instead.