VMS Help UIL, Compile Time Errors, DUP_LETTER  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  ERROR:  Each of the letters used to represent a  color  in  a  color
  table must be unique.  If not, that letter in a icon would represent
  more than one color; each pixel can have only one  color  associated
  with it at a time.  The letter marked has been assigned to more than
  one color.

  USER ACTION:  Choose which color the  letter  is  to  represent  and
  remove or assign a new character to any duplicates.