External document numbers withing the selected documents are not unique

What causes this issue?

The external document number is not unique within the selected
documents.

System Response

The documents could not be transfer posted.

How to fix this error?

Only select documents for transfer posting that belong to the same
external document number.

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.