using
  git format-patch --thread=shallow -o <foo>
and
  git send-email --no-thread --no-chain-reply-to <foo>

duplicates the headers

  In-Reply-To:
  References:

Signed-off-by: Joe Perches <joe <at> perches.com>
---
 git-send-email.perl |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/git-send-email.perl b/git-send-email.perl
index 8ce6f1f..1b9b27e 100755
--- a/git-send-email.perl
+++ b/git-send-email.perl
@@ -1151,7 +1151,8 @@ foreach my $t (@files) {

 	# set up for the next message
 	if ($thread && $message_was_sent &&
-		($chain_reply_to || !defined $reply_to || length($reply_to) == 0)) {
+	    ($chain_reply_to && 
+	     (!defined $reply_to || length($reply_to) == 0))) {
 		$reply_to = $message_id;
 		if (length $references > 0) {
 			$references .= "\n $message_id";
-- 
1.6.3.1.10.g659a0.dirty