Question - How do I resolve error 1520?
Answer - This could mean that there is a duplicate student, or that there are two or more students with no UPN at the school. You may need to check off roll students as well in Admin > Students > Search not on Roll.
Another thing to check is the SEN provisions. Sometimes if a student has more than one open SEN provision or overlapping provisions this can show as multiple student records. To check and amend this click Fix error on the student; go to the support tab and click the cog next to SEN.