Sort a copy of the datafile
    Open sorted copy   
    Until EOD     
       Read line
       Compare to previous line
       If line == previous line
          if line not in duplicate table
              put line in duplicate table
          endif
       else 
          previous line = line
       endif
     end
     Open original data file
     Until EOD
        read line
        if line in duplicate table
           if duplicate not marked
              emit line on output
              mark duplicate line
           end
        else
           emit line on output 
        endif 
     end