What does "duplicates" mean when running the MIS Sync wizard?
If you have run the MIS sync wizard and on the summary page you see a warning saying "possible duplicates" that just means the system has recognised similar names with no unique identifier.
This is usually because you have existing users with no UPN data which is what Purple Mash uses as the unique identifier. (See What MIS data does Purple Mash require? for more details).
You can simply rectify this by expanding the list of "possible duplicates" and selecting resolve for each user identified. If you do not do this, completing the MIS sync wizard will create a new, duplicated user account.