The name of the second enhancement is not relevant.

What causes this issue?

Only one enhancement document is created for the specified enhancement
type.
One of the three following enhancement types was chosen:

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

Overwrite original document
Insert before the original document
Append to the original document

System Response

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.