Invalid change to an alternative hierarchy (see long text)

What causes this issue?

You want to hang group &V1& under group &V2&. However, both groups also
exist in other hierarchies. If you carried out this change, group &V2&
would exist in at least one of these alternative hierarchies more than
once.

System Response

You cannot make this hierarchy change.

How to fix this error?

Check which hierarchies contain groups &V1& and &V2& (using the
where-used list). Change the hierarchies you find so that the change you
want to make to the current hierarchy is then possible.

Error message extract from SAP system. Copyright SAP SE.