I have read of many other people having duplicate email issues on here but none correspond to this very specific issue we are seeing.

We have a Zimbra 7 server deployed with a split DNS configuration for a particular domain (say high.com.au). There are about 15 users / email addresses on this domain.

When user admin@high.com.au sends email to paul@high.com.au, paul receives 2 copies. This does not happen for any other user on the domain (i.e. admin sends to vince, no duplicate. Vince sends to Paul, no duplicate).

The interesting thing is that when looking at the headers of the duplicates, one of them has correct headers and the other is missing the SMTP routing information. To be more specific, below are the two headers. Note the identical message ID. Note also that this is not a client issue as it is being sent / receive via the webmail system. I have also included the corresponding mail.log entry for analysis.

What is causing the duplicate email to come through?
More importantly, how do I stop the duplicate email coming through for this user?

HEADER #1
Return-Path: admin@high.com.au
Received: from zimbra.high.com.au (LHLO
zimbra.high.com.au) (10.0.0.201) by
zimbra.high.com.au with LMTP; Tue, 16 Aug 2011 10:42:42 +0800
(WST)
Received: from zimbra.high.com.au (zimbra.high.com.au [10.0.0.201])
by zimbra.high.com.au (Postfix) with ESMTP id 776398F0128
for <paul@high.com.au>; Tue, 16 Aug 2011 10:42:42 +0800 (WST)
Date: Tue, 16 Aug 2011 10:42:42 +0800 (WST)
From: admin@high.com.au
To: paul@high.com.au
Subject: Test dupes from Admin2
Message-ID: <445a19ab-7394-4e41-8cdc-ffe928d3ce65@zimbra>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
X-Originating-IP: [202.89.177.93]
X-Mailer: Zimbra 7.1.1_GA_3196 (ZimbraWebClient - SAF3 (Mac)/7.1.1_GA_3196)

Test2
----------------------------------------------------------------------------


HEADER #2
Return-Path: <admin@high.com.au>
From: <admin@high.com.au>
To: <paul@high.com.au>
Subject: Test dupes from Admin2
Date: Tue, 16 Aug 2011 10:42:42 +0800
Message-ID: <445a19ab-7394-4e41-8cdc-ffe928d3ce65@zimbra>
MIME-Version: 1.0
Content-Type: text/plain;
charset="utf-8"
Content-Transfer-Encoding: 7bit
X-Mailer: Zimbra 7.1.1_GA_3196 (ZimbraWebClient - SAF3 (Mac)/7.1.1_GA_3196)
Thread-Index: AcxbviuUsRq9j1PdQyuLcvfrX1ZeEQ==
X-OlkEid: D224912BE40329A71FE5874F8AB12F9D00FBB85C

Test2
---------------------------------------------------------------------------

There is only a single line in mail.log when the message is sent and it looks as fine as any other internal message that is sent:

Aug 16 10:42:42 zimbra postfix/smtpd[4081]: connect from zimbra.high.com.au[10.0.0.201]
Aug 16 10:42:42 zimbra postfix/smtpd[4081]: 776398F0128: client=zimbra.high.com.au[10.0.0.201]
Aug 16 10:42:42 zimbra postfix/cleanup[4084]: 776398F0128: message-id=<445a19ab-7394-4e41-8cdc-ffe928d3ce65@zimbra>
Aug 16 10:42:42 zimbra postfix/qmgr[6260]: 776398F0128: from=<admin@high.com.au>, size=669, nrcpt=1 (queue active)
Aug 16 10:42:42 zimbra postfix/smtpd[4081]: disconnect from zimbra.high.com.au[10.0.0.201]
Aug 16 10:42:42 zimbra postfix/lmtp[4085]: 776398F0128: to=<paul@high.com.au>, relay=zimbra.high.com.au[10.0.0.201]:7025, delay=0.25, delays=0.06/0.04/0.01/0.14, dsn=2.1.5, status=sent (250 2.1.5 Delivery OK)
Aug 16 10:42:42 zimbra postfix/qmgr[6260]: 776398F0128: removed