Duplicate Invoice Check for Transaction MIR7

Hi Experts,

My client requirment is .

Currenty system checks the BUKRS,LIFNR,WAERS,XBLNR,WRBTR,BLDAT for duplicate invoice this is a standard SAP functionality for T.code MIR7.

Logic needs to be changed

Duplicate Check in SAP for Vendor invoices to only check Vendor number (LIFNR) and Reference number (XBLNR).

I have found a function module which is used to through the error message CUSTOM_MESSAGE is the FM under MRM_FI_DOCUMENT_CHECK. advise what can be done for this.

Thanks,

vikas