External person ID &1 is already used for another personnel number

What causes this issue?

The system attempts to generate the external person ID in <ZK>infotype
0709</> but determined the person ID &1 because of the generation rule
defined in Customizing.
However, this value is already being used for another person.
Since the external person ID must be unique, the value for this current
personnel number remains empty.

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

System Response

How to fix this error?

Check the personnel numbers that use the same external person ID and
correct the external person ID manually in <ZK>infotype 0709</>.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.