#! /usr/bin/awk -f

BEGIN {
    sep = sep ? sep : " "
    end = end ? end : ""
}

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

END {
    _sep = ""
    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 = sep
    }
    printf "%s", end
}