MigrationWiz was designed to never duplicate items that have been migrated by our service. No matter how many times you submit a migration, you should not encounter duplicates.

But if you do encounter duplicates, it is most likely due to one of the following scenarios:

1. You selected the option Do not search destination for duplicates in the Project configuration. This option will disable duplicate detection.
2. You performed a migration previously with a 3rd party tool prior to using MigrationWiz. We will not check for duplicates against data migrated by a third-party tool.
3. The Source mailbox contains duplicate items. The Destination mailbox will reflect the data contained in the Source mailbox.
4. You changed the protocol used by the connector to connect to the Source, for example from IMAP to Exchange. We cannot prevent duplicates in this case.
5. The Source mailbox is forwarding email to the Destination mailbox. This results in two copies: a forwarded item and a migrated item. 
6. You've enabled specific options to force remigration​ without detection of duplicates, etc.​

Click here for additional information regarding delta migrations.