"Source and Target Domains are equal" error (4247106)

When creating a new Domain Pair and entering the source and target domains with their respective administrator credentials, an error is received stating "Source and Target Domains are equal".

This can be related to a problem in a test  LAB when the actual domains have been created from the same computer image.
When a server is promoted to DC, the domain inherits the computer SID, so if two future DCs were copied from the same image, new domains would have the same SID, which does not normally happen in the real production environment and is not supported.

When such problem is encountered in the test environment it is recommended to delete second virtual domain, copy the original image once again, then run a third-party tool to change computer SID on it (for example newsid.exe http://technet.microsoft.com/en-us/sysinternals/bb897418.aspx) and only then promote the server to DC