UPN duplicates cause issues with Migration Manage - "Error 0xEA000001. Failed to update object in (4340914)

Error received when trying to create a domain pair:

"Error 0xEA000001. Failed to update object in ADAM with objectGUID = (GUID) LDAP error 0x14 Attribute Or Value Exists"

This issue is caused by the fact that the additional UPN suffix is the same as the default suffix. According to the algorithm used by Migration Manager for multi-valued attributes, the tool allows you to input identical items but  eventually fails.

Please remove the suffix from domain suffixes and you should be able to configure the domain pair.

Instructions:

1. Open "Active Directory Domains and Trusts" for source domain.
2. Right-click Active Directory Domains and Trusts in the left window pane and select Properties.
3. Under the tab "UPN Suffixes" review the suffixes and remove the unnecessary suffix.

Perform the same steps in the  target domain.

For more details refer to Microsoft Knowledge Base Article ID: 243629 - "HOW TO: Add UPN Suffixes to a Forest", which contains instruction how to remove/add UPN Suffixes: http://support.microsoft.com/kb/243629/