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

my %hash;

while (my $line = <DATA>)
{
    next if $line =~ /^\s*$/; #skip blanks
    chomp $line;

    my ($value, $key) = split ':',$line,2;
    push @{$hash{$key}},$value;
}

foreach my $key (sort keys %hash)
{
    print "$key => @{$hash{$key}} ERROR Single Node!\n" if @{$hash{$key}}<2;
}

=prints
vg3:38 => node1 ERROR Single Node!
vg3:40 => node2 ERROR Single Node!
=cut

__DATA__
node1:vg1:36
node2:vg1:36
node1:vg2:37
node2:vg2:37
node1:vg3:38
node2:vg3:40