Change the standard function module

The standard function module FI_DUPLICATE_INVOICE_CHECK will check whether an invoice/credit memo has already been posted where all of the following attributes match: Company code, vendor, currency, document date, reference number (If a reference number was entered in the current invoice/credit memo). If no reference number was entered in the current invoice/credit memo the system checks whether an invoice/credit memo has already been posted where all of the following attributes match: Company code, vendor, currency, document date, amount in document currency

my requirement is

The above functionality described in the short description doesn’t provide for the need within AP to check on duplicate invoices. The functionality in FI differs from the functionaltiy provided in LIV and therefor needs to be aligned. The risk of posting duplicate invoice entry is eminent since the check is dependent on all parameters. For a good duplicate invoice number check we need FI to check on Vendor and invoice reference within the same fiscal year. To modify rules, copy FI_DUPLICATE_INVOICE_CHECK, modify the checks and then call the copied function module from FB60. The remaining field check in the standard function module should be deleted.

how to achive this. i didn't understand the requirement .Please give me any suggestion for this.