An admin is working with Order Management licensed as standalone (B2C Commerce is not in...

Two likely causes that could make the second submission of the same JSON payload fail are:

The payment data must be updated. The payment data in the JSONpayload contains information such as the payment method, amount, and authorization code. If the same payment data is used for the second order, it may cause a duplicate payment error or an invalid authorization error.

The Order Reference Number must be updated. The Order Reference Number is a unique identifier for each order that is generated by the external system and sent in the JSON payload. If the same Order Reference Number is used for the second order, it may cause a duplicate order error or an invalid order error.

Verified References: https://help.salesforce.com/s/articleView?id=sf.order_management_order_api.htm &type=5 https://help.salesforce.com/s/articleView?id=sf.order_management_payment_api.htm &type=5