Hello,
Check this Article:
Duplicate SEP clients appear in the Symantec Endpoint Protection Manager console
There are two causes for this issue:
Current Theory: The first possible cause for this is when an Endpoint has been re-imaged (whether in a virtual machine or on a physical system).
Things we know: Each installation of Symantec Endpoint Protection (SEP) randomly creates a "Unique Identifier" for the client. So if this changes and the re-imaged system checks in, it is recognized as a new client.
Example: The IP and computer name are the same, yet the database still shows a different Unique ID.
The second cause for this is related to an issue with moving clients to a different OU in Active Directory.
Duplicate Endpoint Protection client IDs occur if the base image was not prepared for cloning. For more information, read the article How to prepare a Symantec Endpoint Protection 12.1 client for cloning.
However to Resolve this issue, check these Articles for SEP 12.1: -
How to repair duplicate IDs on cloned Symantec Endpoint Protection 12.1 clients
http://www.symantec.com/docs/TECH163349
Duplicate SEP clients appear in the Symantec Endpoint Protection Manager console
To prevent this from happening, I would recommend you to prepare your clients before cloning /image, check this article:
How to prepare a Symantec Endpoint Protection 12.1 client for cloning (image)
http://www.symantec.com/docs/HOWTO54706
Hope that helps!!