Hierarchical vocabularies entry edition/deletion deletes too many entries

When a hierarchical directory has an entry with id=A and parent=P, and another entry with id=A and parent=Q, then deleting one entry deletes both.
This is because internally directory ids are supposed to uniquely identify the entry, and the case of a multi-column primary key was not considered. Only the id is taken into account in the SQL delete statement. (I haven't checked LDAP yet.)