#!/usr/bin/env perl
use strict;
use warnings;

my $passwd = '/etc/passwd';
open my $fh, '<', $passwd or die "Cannot open $passwd: $!\n";

my (%uid_seen, %user_seen, %uid_dups, %user_dups);

while (my $line = <$fh>) {
    chomp $line;
    next if $line =~ /^\s*#/ || $line eq '';
    my @f = split /:/, $line;
    next unless @f >= 3;

    my ($user, undef, $uid) = @f[0,1,2];

    push @{ $uid_dups{$uid} }, $user if exists $uid_seen{$uid};
    $uid_seen{$uid} = $user;

    push @{ $user_dups{$user} }, $uid if exists $user_seen{$user};
    $user_seen{$user} = $uid;
}

close $fh;

if (!%uid_dups && !%user_dups) {
    print "No duplicate UIDs or usernames found.\n";
    exit 0;
}

if (%uid_dups) {
    print "Duplicate UIDs:\n";
    for my $uid (sort { $a <=> $b } keys %uid_dups) {
        my @users = do {
            my %seen;
            grep { !$seen{$_}++ } ($uid_seen{$uid}, @{ $uid_dups{$uid} });
        };
        print "  UID $uid: ", join(", ", @users), "\n";
    }
}

if (%user_dups) {
    print "Duplicate usernames:\n";
    for my $user (sort keys %user_dups) {
        my %seen_uids;
        my @uids = grep { !$seen_uids{$_}++ } ($user_seen{$user}, @{ $user_dups{$user} });
        print "  $user: UID(s) ", join(", ", @uids), "\n";
    }
}