This option to List duplicate pupil UPNs is available to Nominated Contacts and above only.
Selecting this from the dropdown list will produce a read-only list of all pupil accounts that have identical UPN numbers. You will then be able to see whether these belong to a pupil with the same name, date of birth, and possibly MIS GUID and decide how they should be dealt with. The results can be exported to a spreadsheet and each account can then be resolved on an individual basis.

These results can only be viewed by the LA and will show all data, including USO usernames and which school(s) where the duplicate UPNs are found. Results can be generated for all schools at once or any school within the LA that exports data via Auto-Update.
See also:
List when a school last had a file processed
List pupils that have not been matched
Email grace period configuration
List pupils who were created as part of the process
List pupils who were transferred as part of the process
List pupils who were unallocated as part of the process
List staff that have not been matched
List staff who were created as part of the process
List staff who were transferred as part of the process