use warnings;
use strict;

my %groups;
while (<DATA>) {
    next unless /\S/;
    chomp;
    $groups{$_}++;
}

for (sort keys %groups) {
    print "$_\n" if $groups{$_} > 1;
}

__DATA__
Group 1

Group 2 

Group 3
Group 3

Group 4

Group 5 
Group 5 
Group 5 

##</code><code>##

Group 3
Group 5