list

   1 #! /usr/bin/awk -f
   2
   3 ! /^\#/ && ! /^$/ {
   4     xs[$1]++
   5 }
   6
   7 END {
   8     for (x in xs) {
   9         if (xs[x] > 1) {
  10             printf ">>> [WARNING] : '%s' was listed %d times.\n", x, xs[x] > "/dev/stderr"
  11         }
  12         printf("%s%s", sep, x)
  13         sep = " "
  14     }
  15 }