SID are used to uniquely identify each object in the Windows world . An analogy will be social security numbers or NRIC number.
I was very surprised when I heard that the famous sysinternal tool, newsid has ben depreciated. Afterall you need to use when deploying clones.
It seems that having identical SIDs does not affect security operations., except when you are talking about domain controllers. Hmmm. Hit the link here.