Multiple Notices for an already filed return

I submitted my 2015 tax returns in 2015 and everything was finalized on time without error. Now, in 2017, as I am about to submit the 2017 one, I get a notification on E-filing saying my 2015 return is outstanding. I resubmit the same return without changes and get a notice of assessment saying I owe SARS around R3500 for a finished return. I call the call centre and they tell me to lodge a dispute. Dispute lodged and supporting documents submitted. Today, I get another Notice of Assessment saying I owe SARS a futher R1130 or so FOR THE SAME ASSESSMENT!!! Why? Why am I getting multiple notices for returns completed and finalised in 2015? And why do I keep on owing SARS money that is inexplicable? Your system has a problem in that it can't pick up returns filed previously despite the record of return being there, and then it miscalculates the same returns. Please fix this before you sequestrate me over money I don't actually owe you.