We can view this by following these steps

Navigate to:
  1. Core
  2. Users
  3. Edit User Profile Data
  4. Search for the User
  5. Click on the Users Name
  6. Scroll down to System Information and click on Alumni Affiliation
They have 2 shown here. Student's should only be associated with one grad year. This is the reason the report is duplicating students.

     8.  Click Edit on the year of graduation that is not correct and Click Delete