A great article by Mark Russinovich (the creator of the tool NewSID), about the Myth around duplicate Security Identifiers. The NewSID tool is frequently used by System Administrators to change a computer’s SID, for cloning purposes, after you read this article you get a better understanding about SID’s and why the tool has been retired

http://blogs.technet.com/markrussinovich/archive/2009/11/03/3291024.aspx