use strict;
use warnings;
my %seen;
print map  { $seen{$_} > 1 ? $_ x $seen{$_} . "\n" : () }
      grep { not $seen{$_}++ }
      grep { !/^\s*$/ } <DATA>;
__DATA__
Group 1
 
Group 2
Group 3
Group 3
Group 3

Group 4

	
Group 5
Group 5


Group 5
Group 5
Group 1