Specification is already being used reference specification

What causes this issue?

You attempted to assign other references to a specification that is
already being used as a reference.

How to fix this error?

In the hit list for specification management under <LS>Utilities ->
Where-used list</>, check which specification this reference is
assigned to.
Assign the other references to the selected referencing specification
directly.

Error message extract from SAP system. Copyright SAP SE.