Hi Aspose Team,
While sorting the data in ms excel, if any two sort keys are given for the same column then an error "The sort reference is not valid. Make sure that its within the data you want to sort, and the first sort by box isn't the same or blank" is thrown. Please let us know if this is expected behaviour in Aspose.Cells as well. Please refer the attached document for the screen shot.
Thanks,
Muin