I ran into a problem where duplicate SIDs existed and caused a lot of problems, because i have cloned a base installation of Windows 2008 R2 server.
Not a good idea if the Domain Controller and an Exchange 2010 server have duplicate SIDs.
So, quick tip:
Never, never, never forget to run Sysprep on a cloned machine:
NOTE: Don’t forget to check the “Generalize” checkbox, or the SID will not be resetted!)