Decommissioned Domain controller

If you have decommissioned a Dc, but it will shown under active directory sites and services. We need to manually delete that server from the list. There will be no NTDS settings underneath decommissioned server.

Same SID of my virtual machines

I copied virtual machines installed server 2012 on them and when I tried to add other VM into the domain of first one. There was a error in joining the domain which was same SID of the machines.

Run the sysprep tool from your C:\Windows\System32\sysprep folder

Also, to use this tool, machine should be in workgroup not in a domain. if it is already in a domain, then it will joined to workgroup and you will not be in that domain. you need to join domain again after using this tool.

Please keep in mind, that you may need to reactivate the windows after using this tool.