VREG, chapter 8 (Routines), page 110

     Let  `el'   be   an   Entity_declaration_list.    Let
     `identifiers'   be   the   concatenation   of   every
     Identifier_list of every Entity_declaration_group  in
     `fa'. Then `el' is valid if and only if no Identifier
     appears more than once in the list `identifiers'.