I’ve migrated users from several Exchange organizations (Exchange 2003 and 2010) into a single organization running Exchange 2010. Each migration has had its own quirks. This one was curious:

Migrating from Exchange 2003 to an existing Exchange 2010 organization. We used Exmerge to export the mailboxes from the 2003 system to pst files, then used New-MailboxImportRequest to import them to the 2010 system. When the end users logged into their 2010 mailboxes, some of them had duplicates of some of the managed folders, specifically Sent Items, Deleted Items, Junk E-mail, Suggested Contacts, and Sync Issues. The folders imported from the pst file kept their original names, while Exchange 2010 created Deleted Items1, Junk E-mail1, etc. with the system-created folders being used as the actual managed folders. So new junk e-mail would go to Junk E-mail1 instead of to Junk E-mail. This was the third 2003 -> 2010 migration I’ve done, but the first time I’ve seen this. We had to rename the imported folders if the user needed to preserve the contents, then use MFCMAPI to delete the new …1 managed folders and let the system recreate them with the correct names.

I’ve read several sources that say this can happen if the original mailbox and the new mailbox had different language settings. For example, if the Exchange 2003 mailbox was using French and the Exchange 2010 is set to en-US (US English). That was not the case here. All clients and mailboxes are using US English. Right now, it appears that the users who had duplicate folders were using a 64-bit client of Outlook 2010.