One way is to keep track of the group count in a hash: 
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

[download]
Prints... 
Group 3
Group 5

[download]

In reply to Re: Shorten a list... by toolic
in thread Shorten a list... by la

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