When you get multiple copies of an email message, the easiest way to find out who is duplicating it is to look for when the headers start to change.
If the two messages have different Message-ID headers, then the duplication is happening on the sender's computer. It could be that they accidentally sent it twice, or that their computer lost its connection to the Internet while sending, and the program sent it twice on their behalf.
If the two messages have the same Message-ID header, though, then you should start looking at the Received: headers. Specifically, start checking the logging ID. For example, in the Received: header below, the logging ID is 8D5212E7B1.
Received: from a-icg-mx-sd.icgroup.com (a-icg-mx-sd.icgroup.com [220.127.116.11]) by lab.pobox.com
(Postfix)with ESMTP id 8D5212E7B1 for <address>; Mon, 13 Jun 2011 08:15:26 -0400 (EDT)
Every logging ID is unique for a message. So, when reviewing Received: headers, if the logging ID matches, that means your two messages were one when they passed through that machine. The first Received header:, as read from top to bottom, where the logging ID is same is the computer that caused the second copy. They should also have logs that indicate why or how the message was duplicated.