I was building /upgrading my LAB environment to windows 2008 R2 / server 2012.
When I was adding members to the Local Administrator group, member gets added but than disappears from the view, I tried on different server as well, same behavior.
Suddenly I realized that I have cloned them without changing the SID :) , identical SID issue ..
Old problem but worth to be shared..
So, What We have to do is : On the machine from which you want to clone..
On the command prompt enter below (we have to use sysprep so that machine SID can be changed)
C:\Windows\System32\sysprep\> sysprep.exe /oobe /generalize /shutdown
Now when I have prepared machine cloning from this machine, below is the result on members servers.