use strict;

undef $/;
my %seen;
my @para          = split /(\n\n+)/, <>;

while (defined($_ = shift @para)) {
    die "No From line!\n" unless /^From /;

    my ($id)      = map /^Message-ID:\s(\S.*)/im, 
                    split /\n(?! )/;

    warn "No Message-ID! [[$_]]\n" unless defined $id;
    $_ .= shift @para while @para and $para[0] !~ /^From /;
    print unless defined $id and $seen{$id}++;
}