External document &1 is already being used

What causes this issue?

You have already processed an intermediate document (IDoc) for
external document number &V1&. Either the IDoc has been submitted more
than once or there are several IDocs for the same external document
number.

System Response

The system does not process an IDoc if it does not have a unique
external number. If the system allowed several IDocs for one external
number, you would not be able to determine a referenced document for
correction documents, for example.

How to fix this error?

Change the external document number in the IDoc, or ask the customer to
send a correct IDoc.

Error message extract from SAP system. Copyright SAP SE.