To identify potential duplicate users in Genius, we have a Duplicate Users report that will find users with the same name or email address. Once identified, you can choose to keep both accounts, keep the left account, or keep the right account. All enrollments will be merged into the user account you designate to keep.
In the LMS, you will need make that merged account unavailable so the user can no longer continue to use it. All future enrollments should be from the newly merged user account.
If there are more than two matched user accounts that need to be merged into one, it is best to merge them one at a time.