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There are two settings in AP Setup to warning on duplicate invoices for vendors.  You can select that you want a warning on the vendor id, and amount, or vendor id, and invoice number, or use both.  Also, there is a setting in AP Setup to say how many periods you want it to keep warning you on duplicates that you must set.  

Keywords: duplicate,warning,accounts Payable, voucher,both,invoice number,vendor id