$hdr->header_set('Message-ID', $mid0);
my @alt = $hdr->header('X-Alt-Message-ID');
my %seen = map { $_ => 1 } (@alt, $mid0);
- foreach my $m (@mids) {
- next if $seen{$m}++;
- push @alt, $m;
- }
+ push(@alt, grep { !$seen{$_}++ } @mids);
$hdr->header_set('X-Alt-Message-ID', @alt);
}