my %seen_users;
for my $user (@users) {
    next if $seen_users{$user}++;   <-- Add this line
    ...
}

##</code><code>##

my @users = qw( John Jane John Jeff );

my %seen_users;
for my $user (@users) {
    next if $seen_users{$user}++;
    print "$user\n";
}

##</code><code>##

John
Jane
Jeff