VMRC, chapter 11 (Repeated inheritance), page 191

     It is  valid  for  a  class  `D'  to  be  a  repeated
     descendant  of  a  class  `A'  if  and  only  if  `D'
     satisfies the  following  two  conditions  for  every
     feature `f' of `A':

        1 If the Repeated Inheritance  rule  implies  that
          `f'   will  be  shared  in  `D',  then  all  the
          inherited versions of `f' are the same feature.

        2 If the Repeated Inheritance  rule  implies  that
          `f'  will  be  replicated  in  `D'  and  `f'  is
          potentially ambiguous, then the Select subclause
          of  exactly one of the Parent parts of `D' lists
          the corresponding  version  of  `f',  under  its
          final `D' name.