debugging School Data Sync errors (SDS)

[supportman]

I'm trying to import my classes to office 365 this year and am recieving this error

 

EntryStatus,EntryType,ErrorCode,ErrorMessage,JoiningValue,MitigationSteps,Operation,RecordedTime,TenantActionable,ReportableIdentifierError,
Student,DuplicateTargetEntries,Two or more students exist with the same SIS ID in Azure Active Directory.,10960,,Add,9/11/2018 9:26:56 AM,True,10960

 

Does anyone know what DuplicateTargetEntries means? and how to fix?

 

I've found this list of errors but can't work out what it actually means. Their explanation of "Two users exist with the same SIS ID in the source directory " makes no sense to me.

 

https://docs.microsoft.com/en-us/schooldatasync/school-data-sync-errors-and-troubleshooting