Since they are current students, they do not need to go through the checklist or application process and you will want to delete their erroneous profile.
- Go to Core > User/Access > Profiles
- Search for the student
- Click the Profile for the Active Candidate
- Scroll Down to System Information
- Click Remove User
- Click Delete
- Click Ok
Once you delete the duplicate profile, the duplicate application and checklist will also be removed.