During the installation of Exchange 2010, the computer account for the Exchange Server is added to the Exchange Server security group. This ensures that Exchange 2010 on that server has access to any necessary information in Active Directory.
In most cases, computer names are unique between domains. However, if the computer that Exchange 2010 is being installed onto has the same computer name as a computer in another domain in the forest, the process of adding the new Exchange 2010 server to the Exchange Servers security group fails and the installation of Exchange 2010 fails.
This error occurs based on an Active Directory search for the computer name. So, the computer account with the conflicting name may be left over from a long time ago.
Easy solution, if the conflicting computer account is no longer required, then remove it. If the conflicting computer account is required, then rename your server before installing Exchange Server 2010.