#!/usr/bin/env perl

use strict;
use warnings;

my ($host, %seen);
my $host_re = qr{ \A ( hostname \d+ [.] com ) \s* \z }msx;
my $fail_re = qr{ \A ( .*? ) \s*? \z }msx;

{
    local $/ = "";

    while (<DATA>) {
        if (m{$host_re}) {
            $host = $1;
            print;
        }
        else {
            print unless $seen{$host}{(m{$fail_re})[0]}++;
        }
    }
}

__DATA__
hostname1.com

Gateway FAIL

Gateway FAIL

Gateway FAIL

hostname2.com

Gateway FAIL

Gateway FAIL

Gateway FAIL