The field & has already been assigned to the SAP code

What causes this issue?

Each field with an SAP code must be assigned to a field with an ISO
code.
The ISO code field &v1& cannot be assigned to another SAP code because
it is already assigned to the SAP code field &v2&.

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?

Choose a different ISO code field. If this is not possible, the
external structure is incorrect and another ISO code field should be
included.

Error message extract from SAP system. Copyright SAP SE.