I am seeing two copis of every outbound email in the Sent items

I am seeing 2 copies of every email I send through my SMTP client in the Sent items folder.

I opened the headers on them and one seems to be the incoming message (with the headers showing they came fomr my machine) whereas the 2nd one appears to be one it has been processed by Zohomail (one is the incoming to the server, whereas the 2nd one is the outbound copy)

Header from first message:

From: "XXX" <XXX@XXX.XXX>
To: <XXX@YYY.YY>
Subject: Follow up email
Date: Sat, 9 Nov 2013 11:14:43 -0700
Organization: XXX
Message-ID: <005301cedd77$91c11540$b5433fc0$@XXX.XXX>
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="----=_NextPart_000_0054_01CEDD3C.E562D980"
X-Mailer: Microsoft Outlook 14.0
Thread-Index: AQJjjxdWyTe1gMrTKUdTiFvasA60lg==
Content-Language: en-us
X-OlkEid: 5E640320C5F041EDA2F1E843A70A0231A4E900A4

Header from second message:

Received: from XXX (XXX.XXX [XX.XXX.XX.XX]) by mx.zohomail.com
with SMTPS id 1384020885070543.5489579460955; Sat, 9 Nov 2013 10:14:45 -0800 (PST)
Reply-To: <aXXX@xx.xxx>
From: "xxx" <xxx@yyy.yyy>
To: <yyy@yyy.yyy>
Subject: Follow up email
Date: Sat, 9 Nov 2013 11:14:43 -0700
Organization: XXX
Message-ID: <004e01cedd77$9159c7f0$b40d57d0$@xxxx.xxx>
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary="----=_NextPart_000_004F_01CEDD3C.E4FB8C30"
X-Mailer: Microsoft Outlook 14.0
Thread-Index: Ac7ddKgZVxR6Dr6zQIW5nOQUYpOJEg==
Content-Language: en-us
X-ZohoMailClient: External
X-Zoho-Virus-Status: 2

This is a minor quirk. Any ideas what is causing this? I have verified that only the one with the "received from" header is being sent to the recipient.