Duplicate, duplicates, duplicates
My current project has me consolidating almost 50 Windows NT 4.0 domains and various Exchange 5.5 sites and organizations in to a single Active Directory domain / Exchange 2003 organization.
At the beginning, we were told that all user and Exchange aliases accounts would be renamed to a standard. Right before the migration, we were told that would not happen. We scrambled and sent out lists of potential duplicates of accounts, computer names, groups, distribution lists, and Exchange aliases. Not all of these have been cleaned up.
As a result, during each block of user migrations, we end up with duplicates. We import mailbox information programmatically and we end up with duplicate Exchange aliases and, worse, SMTP addresses. We are having to run daily checks for duplicate SMTP addresses! (Thank goodness for Rich Mathesien's DUPSMTP.VBS checker!)
In highsight (and, as my mother would say, hindsight is ALWAYS 20-20), we should have waited until we had triple validated all potential duplicates before proceeding with the migration.
Always fix your duplicates before migrations!!!!!