Rest[Union[Riemann6ab /. {-1 -> 1}]]

Out[53]=

          ab       cd   a b       ce   a b        c   abd
  {R R   R  , R   R   R    , R   R   R    , R   R   R    , 
      ab       ab      c d    ab      c e    ab  d   c
   
          c   abe       cd   b a       ce   b a
    R   R   R    , R   R   R    , R   R   R    , 
     ab  e   c      ab      c d    ab      c e
   
          c   bad        c   bae       de   a b
    R   R   R    , R   R   R    , R   R   R    , 
     ab  d   c      ab  e   c      ab      d e
   
          d   abc        d   abe       de   b a
    R   R   R    , R   R   R    , R   R   R    , 
     ab  c   d      ab  e   d      ab      d e
   
          d   bac        d   bae        e   abc
    R   R   R    , R   R   R    , R   R   R    , 
     ab  c   d      ab  e   d      ab  c   e
   
          e   abd        e   bac        e   bad
    R   R   R    , R   R   R    , R   R   R    , 
     ab  d   e      ab  c   e      ab  d   e
   
             acbd           acbe           adbc
    R   R   R    , R   R   R    , R   R   R    , 
     ab  cd         ab  ce         ab  cd
   
             adbe           aebc           aebd
    R   R   R    , R   R   R    , R   R   R    }
     ab  de         ab  ce         ab  de

Up to A Very Large Scale Pattern Matching Problem in MathTensor