Serial number &1 already exists for material &2

What causes this issue?

You are trying to create <GL:serial_number>serial number</> &V1& for
the <GL:material>material</>.
This combination of <GL:material_number>material number</> and serial
number already exists in your system.

System Response

INCLUDE 'KEINE-FUNKTION' OBJECT DOKU ID TX LANGUAGE E
Your system has been configured in such a way that the combination of
material number and serial number must be unique. Therefore, you cannot
assign a serial number to a material twice.

How to fix this error?

Enter a serial number that has not been assigned for this material.
If this is not possible, contact your system administrator.

Error message extract from SAP system. Copyright SAP SE.