What Katrina is seeing makes sense. Under AP options setup, look to see what the number of days to retain open invoices is set to. It sounds like the invoice was entered and paid previously but time has passed and the invoice was purged out of the AP open invoice file. For some reason, the vendor is reusing the numbers (maybe it is a statement # or a telephone bill) The invoice is retained in the Invoice HISTORY table until a purge routine is manually run. So you can either purge the invoice history (I wouldn't) or add a character to make the invoice # unique. It sounds like it is working properly. I would just add another character and move on.