Resource from HR already exists as & in plant &

What causes this issue?

You want to create a resource by referencing a resource maintained in
the Human Resources System. However, you have already referenced this
resource when you created another resource.

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?

Reference a resource from the Human Resources System which has not
already served as a reference or
create a resource without referencing a resource from the Human
Resources System.

Error message extract from SAP system. Copyright SAP SE.