Mailbox Migration from 2003 to 2010 fails for one user

Hi

Having an issue with a single mailbox that will not move from the 2003 server to the 2010 server.

The move itself gets to 95% but when it is being finalized it fails with

Error: An error occurred while updating a user object after the move operation. --> Active Directory operation failed on dc2.domain.com. One or more attribute entries of the object 'CN=User Name,CN=Users,DC=domain,DC=com' already exists. --> The attribute exists or the value has been assigned.

All permissions on the account are ok.  No corrupt x400 address.

Exchange 2010 SP2 RUP4

This is the only mailbox that will not move across.

I can post the log if required - its quite long so that is why i haven't attached it here

Thanks

Joe