Vocabularies must not allow creation of duplicate entry id

When creating a vocabulary entry, one must not allow the creation of an id that already exists.
Display an error message (like "The identifier already exists" / "L'identifiant existe déjà") if that's the case.

But be careful for hierarchical vocabularies, as in that case the unique thing is the couple (parent, id). (There may be several identical ids but with different parents.)