GSTR1 error - invoices already submitted

I am trying to file gstr1 return for July 2017 on system portal.
I have saved and successfully submitted the data.
But when i try to file the return with EVC option and after entering the OTP received, the message come on the screen is " Error, invoices already submitted". What does that mean ?
I have tried this for 5_6 times but every time same error pops up.