If you import a PST file or mailbox and the import does not complete, you can import the same PST or Mailbox again and it will detect the previously uploaded items and not create duplicates. This is with the default Duplicate Detection of PSTId selected. This also applies if you are importing data from on on premise Exchange server. The big caveat with the on premise Exchange Migration is if a user moves or deletes messages, those changes will not be synchronized to the user's Office 365 mailbox, if a later migration is run against their mailbox.
If you have already attempted an import with Outlook, and the messages in the PST file originated from Exchange, you must choose the SourceKey option in the Duplicate Detection area on the options screen to prevent duplicates from being created. If the messages did not originate from Exchange, for example they came from a POP3 server, then duplicates would be created if you first tried to import with Outlook and then tried to import with the Migrator.
Updated: April 16, 2016