If you have historical payment transactions perhaps from a previous shopping cart and you're now in the process of configuring the Storefront, you may not be able to process new payment transactions because your payment processor forbids duplicate order numbers in their system. You may encounter error messages such as "The transaction was not sent to the host because of a duplicate order id". The error occurs simply because you have already submitted a previous order with the same order number in the past. For security reasons, certain payment processors will simply reject the new transaction.
In this case, you may want to jump start the next order number sequence. To do so, you must first enable the Sales order feature under Configuration > General. Once enabled, you can head over to Configuration > Sales order screen and change the Next sales order number sequence number. Please note, once you increased the number, you cannot decrease it so please choose an appropriate start number carefully.
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