ABOUT GSTR-1 Problem

I have received message while filing R-1 for Aug 17 as " Inv no.--- is already exists in GSTR-2 of the receiver for the month 072017.Enter correct Inv number" actually my client has issued invoice Dt 02/08/17 but customer has claimed wrongly in July 17. Customer has filed R-1 & R-2 for July 17. now how can I reject this invoice? my R-1 Also submitted. I am unable to file Aug 17 R-1. Pls guide