Billing system not allocating payments correctly

my contract with cell c is due for upgrade, I get in touch with call centre only to be informed that my account is in arrears after i have been paying monthly instalment without defaulting. suprisingly for month of may i got debited twice. 1 my bank statment all debit orders went through with no reversal or any messages reflecting that cell c did not receive thier funds. 2 the last monthly billing statement of April indicates my account is on order no missed payments. So now if this is the case i need this resolved, the debit order that went through on 25th May to be returned to my account.