NMI Error 300: Duplicate Transaction

What This Means

By default, NMI rejects any duplicate transaction within a 1200 second (or 20 minute) window. A duplicate transaction is any transaction executed with the same amount and card number.

Suggested Next Steps

You may check for more information on this specific transaction and decline within your NMI web interface by visiting Reports >> All Transactions.
If the transaction was completed and the payment was successful, you should check to see if the Scheduled Order did actually process successfully.
If the payment was successful, you may Complete Processing the Failed Scheduled Order and check the next occurrence is set to the correct future date.
If the transaction was not completed and paid successfully in NMI, and you wish to retry processing the payment, you should wait at least 20 minutes from the time of the last processing attempt and then Restart Processing the Failed Scheduled Order.
To process at a later time you may edit the Scheduled Order, set the next occurrence date to the desired date, set its status to Active (by "Activating"), and then update the Scheduled Order.
If the issue is persisting and you wish to work around it, NMI suggests that you can enable the option to "Include Order ID when performing Duplicate Transaction check" in the "Advanced Merchant Features" of the merchant setup in your NMI gateway.

After Resolving the Issue

As outlined above, you will either need to Restart Processing the Failed Scheduled Order.