OPDC API error code

DUPLICATE_TRANSACTION

HTTP status 409

A transaction number already posted with different lines. 409.

An identical replay is not an error: it returns the prior result.

The API answers with a problem document (RFC 9457) whose type is https://twinn.engineering/errors/duplicate-transaction, and with X-Error-Code: DUPLICATE_TRANSACTION. Branch on the code, never on the detail text.

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