Check for duplicate Security Identifiers (SID) on your domain

Open a command prompt and type

C:\ ntdsutil

 and press ENTER.

Now type

C:\ security account management

and press ENTER.

Now type

C:\ connect to server DNSNameOfSAMDatabaseServer

 and press ENTER.

Now type

C:\ check duplicate sid

and then press ENTER.

If you have duplicates it will be listed.