Double Billing

Called numerous times to complain about double billing on my tablet account (Device Payment)***) and given a Ref NO. 623809. Last month my bill was even worse (double the amount) I usually pay. I was advised to R1,020 by Colin in Collections Department. But I was told I am responsible for the whole amount despite the fact that this is a CLOSE contract with My Limit on to avoid exceeding the 2GB I get monthly. Can someone get to the bottom of this query? My New Ref No. is 696866