The combination MPN & and Manufacturer & is not unique

What causes this issue?

At least one material master already exists in which the same
combination of MPN and external manufacturer appears.

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?

Via F4 help determine the corresponding material master record and
change the fields MPN and external manufacturer. If any other material
is marked for deletion, material reorganization must be carried out or
the fields MPN and external manufacturer must be changed in the
material marked for deletion, before this combination can be used
again.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.