Internal error within BAPI interface: The interface ID is not unique

What causes this issue?

Internal error in BAPI interface: interface ID not unique.
When creating deliveries for a preceding document from another system,
the request line identification in the <ZH>REQUEST</> segment of the
relevant BAPI interface is not unique.
A value in the REQUEST-ID field must only occur once in the entire
REQUEST table.
This must be ensured by the caller, since this must not be changed in
the BAPI. It must not be changed, because this ID is used to create a
link between the requested delivery item and the error log.

System Response

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

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.