The activity '{1}' cannot reference activity '{0}' because activity '{0}' is already referenced elsewhere in the workflow ...

The activity '{1}' cannot reference activity '{0}' because activity '{0}' is already referenced elsewhere in the workflow and that reference is not visible to activity '{1}'.  In order for activity '{0}' to be visible to activity '{1}', it would have to be a child or imported child (but not an implementation child) of activity '{2}'.  Activity '{0}' is originally referenced by activity '{3}' and activity '{1}' is in the implementation of activity '{2}'.