This is a rare issue. Duplicate pupil records can be created if the Pupil Match function fails to recognise pupils and they are all marked as new, or if the Pupil MIS ID changes randomly, something which may affect hosted SIMS schools (see separate article relating to Admission number).
If the duplicate pupils appear in only 1 series then the quickest way to resolve this is to delete the affected series and re-import it, ensuring that the Pupil Match works correctly. If you are a hosted SIMS school, please download the latest reports from www.4matrix.com/simsreports to ensure that the match will work correctly.
If your duplicate pupils appear in more than 1 series then please see article 206 "Merging Pupils"