If you have duplicate student records in SIMS these will transfer to Arbor as two separate students profiles. In SIMS, you can check for duplicates and delete these so that when you migrate there is only one record created for the student.
Step 1 - Locate the student in SIMS
You can search for the student by name and change the status from Current to Any. This will display any records you have for the student. Any duplicates will likely not have a class or registration group assigned if they are a duplication of a student on roll.
Step 2 - Take note of the SIMS Admission and Delete the duplicate record
Once you have confirmed the student is a duplicate with no attendance taken you can delete the student. You will need to take note of the SIMS admission number and then go to Routines > Pupil > Delete Pupil and cross the SIMS admission number.
If you have already migrated to Arbor, please see our guidance here: Delete or merge a student profile