A payment with this reference is already open or paid

reference is your own identifier for the thing being paid for, and paygate enforces that it is unique per tenant and mode across payments that are created, requires_action, processing or succeeded. A payment that expired, failed or was canceled releases its reference.

What to do

• If you meant to retry the same call, retry it with the same Idempotency-Key instead. Idempotent replay returns the original payment; a new key with a used reference is what produces this error.
• If you meant a new payment for the same order (the payer abandoned the first and came back), fetch the open payment with GET /payments?reference=<your reference> and show its next_action again rather than creating a second one.
• If the first payment should be abandoned, cancel it — payments.cancel_not_allowed covers when you cannot.