Check the following - 

1. This error could mean that there is a duplicate student profile. Use the search box in the top right of your ScholarPack to search for the student.  If there is more than one profile you will need to delete one of them.  Note: Check the data in both the profiles and transfer anything needed before doing this as it cannot be undone.

2. Check the SEN provisions. Sometimes if a student has more than one open SEN provision or overlapping provisions it can show as multiple student records.

To check/amend this click 'Fix Error' which will open the student profile, go to the support tab and then click the cog next to SEN. 

3. There may be two or more students with no UPN at the school.  Go to Admin > Students > Bulk UPNs to see any students with no UPN.

You may need to check off roll students as well in Admin > Students > Search Not On Roll to see if there is a student off roll who also has a live profile, both with the same UPN.

Set the Limit Results To "All Students" and click on Choose. You can then use one of the filters like, for example UPN, then press enter to search for a duplicate.