Dear Friends,now this problem "Invoices already submitted" has gone.I can file GSTR-1 properly.So please try again.If same thing will take place,then be patient and try tomorrow.
I am not sure but it could be the case of wrong coding of software because even the OTP box says enter OTP for submission, where as it should be "Enter OTP for filing return". looks like on pressing File with EVC button, the OTP box of submitting invoices comes up, that's why it gives error "Invoices already submitted".