Invoice Number replaced

Hi,

     I know it is not possible for the invoice numbers generated during the invoice print run, to be replaced with another number. However, I noticed about 8 invoices that were raised on the 10/08/2015 seems to been have replaced with a fresh set of numbers. I could not find a single trace of the original invoice number except the on the printed Invoice and in the S.O. Detail Ledger File (F42199) where I found three lines per item with three different invoice numbers, first being the one that was printed on the Invoice copy. Time of the Day (TDAY)