#! /usr/bin/awk -f

! /^\#/ && ! /^$/ {
    xs[$1]++
}

END {
    for (x in xs) {
        if (xs[x] > 1) {
            printf ">>> [WARNING] : '%s' was listed %d times.\n", x, xs[x] > "/dev/stderr"
        }
        printf("%s%s", sep, x)
        sep = " "
    }
}