Update ap invoice dff by using (ErpObjectDFFUpdateService) Soap Webservice

I have created an oic integration that use (ErpObjectDFFUpdateService) Soap Webservice to Update Payables invoice DFF and return status 0 or 1 But if Invoice Number is Duplicated i can't Update a Specific Invoice however i'm using

Key A: Invoice Number

Key D: Invoice ID,

my Problem is how to update Specific Payables invoice if Exist more than Invoice with the Same Invoice Number