Overcharged for my payments

I have been having a contract with mtn since 2012. I was recently in arrears and handed over to mbd collections. I use to have debit orders of mbd going off my account. In March I went to a mtn store to settle my account and they gave me an account. Now mbd says I still owe Mtn and I should carry on paying them. My itc even reflects that my account is now 0. Now why am i paying MBD for debt mtn who handed me over doesnt know about it.