On some occasions you may see duplicate mail messages or appointments on target mailboxes.
This happens if the mailboxes which are under the scope of the migration are manually synchronized by using Exmerge utility or are copied manually with the .pst method. They are then added into a mailbox collection.
If mailboxes which participated in a mail sync collection for any reason are manually synchronized by the Exmerge utility or manually copied using the .pst method, such mailboxes should be Excluded from any mail sync or calendar sync collections.
Steps must be taken to ensure if they exist in any collection they must be removed and changes to the collections are committed.