Error when creating document flow for delivery &

What causes this issue?

As the system was recording the document flow for the goods movement,
it determined that the subsequent document number is either blank or
identical to the number of the preceding document.

System Response

In order to avoid inconsistencies, the goods movement posting was
cancelled.

How to fix this error?

Make a note of the steps that led to this system response and contact
your system administrator.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.