ParentPay will provide the Student name and Student ID causing the problem so you can check your data.
If you go to the student profile, and click on the Support Tab, you will need to check the dates entered for SEN Provision. Click on the Cog next to SEN in order to see the table with the start and end dates.
There ought to be no future dates entered as an end date, and no overlapping dates (so the start date of one SEN provision ought not to be the same date as the previous one ended). If there are overlapping dates the software perceives the student as being duplicated, which is why it is necessary to ensure the dates are in order.
Click on Edit next to any SEN Provision where the dates require amending. Once saved, you will be able to export the data to ParentPay.