lib/PublicInbox/ContentId.pm | 16 +++++++++++++++-

diff --git a/lib/PublicInbox/ContentId.pm b/lib/PublicInbox/ContentId.pm
index 65d5a76c09b43f4dec2edb08998c22ffe6d6e95e..7ec638ca5ef2ece2eb95912d126338e22f851d3b 100644
--- a/lib/PublicInbox/ContentId.pm
+++ b/lib/PublicInbox/ContentId.pm
@@ -11,7 +11,7 @@ # not sure if less-widely supported hash families are worth bothering with
 use Digest::SHA;
 
 # Content-* headers are often no-ops, so maybe we don't need them
-my @ID_HEADERS = qw(Subject From Date Message-ID References To Cc In-Reply-To);
+my @ID_HEADERS = qw(Subject From Date To Cc);
 
 sub content_id ($;$) {
 	my ($mime, $alg) = @_;
@@ -19,6 +19,20 @@ 	$alg ||= 256;
 	my $dig = Digest::SHA->new($alg);
 	my $hdr = $mime->header_obj;
 
+	# References: and In-Reply-To: get used interchangeably
+	# in some "duplicates" in LKML.  We treat them the same
+	# in SearchIdx, so treat them the same for this:
+	my @mid = $hdr->header_raw('Message-ID');
+	@mid = (join(' ', @mid) =~ /<([^>]+)>/g);
+	my $refs = join(' ', $hdr->header_raw('References'),
+			$hdr->header_raw('In-Reply-To'));
+	my @refs = ($refs =~ /<([^>]+)>/g);
+	my %seen;
+	foreach my $mid (@mid, @refs) {
+		next if $seen{$mid};
+		$dig->add($mid);
+		$seen{$mid} = 1;
+	}
 	foreach my $h (@ID_HEADERS) {
 		my @v = $hdr->header_raw($h);
 		$dig->add($_) foreach @v;