S08 DUPLICATE ISF TRANSACTION

we can refer to ISF Filing Error Codes & Notification Codes to find the rejection reason of ISF filing, which shows as below:

S08 Duplicate ISF Transaction: A stand-alone Add transaction is submitted using an ISF Importer and Bill that was previously submitted.

So likely the ISF had been filed for the same AMS B/L in the past, you can check with your software provider to check whether the ISF had been filed for the same AMS B/L.