Do you care about order? If not, use a hash.

my %data;
while (<>) {
   my $net = (split ' ')[0];
   push @{$data{$net}}, $_;
}

foreach my $group (values %data) {
   if (@$group > 1) {
      print @$group;
   }
}
[download]

Update: Tested. Added missing if.