SID stands for Security Identifier and is used within NT/2000 as a value to uniquely identify an object such as a user or a group. The SID assigned to a user becomes part of the access token, which is then attached to any action attempted or process executed by that user or group. If a duplicate SID did exist then all users with this SID would authenticate as what would be seen as the same user. It is possible for cloned machines to have the same SID, which would be seen by the authentication mechanism as the same machine. The SID under normal operation will be unique and will identify an individual object such as a user, group or a machine.
A SID contains: