Spec. is referenced by &; its reference & already assigns values to '&'

What causes this issue?

The chosen specification is used as a reference for specification &V1&.
The referencing specification &V1& has another reference &V2& that is
already used to assign values to value assignment type &V3&.
If you also assign values to value assignment type &V3& for the chosen
specification, this will cause a conflict because the specification
would then have two value assignment references.

System Response

The system cannot assign values to the selected value assignment type.

How to fix this error?

Resolve this conflict by using a different reference or by choosing a
different value assignment type.

Error message extract from SAP system. Copyright SAP SE.