I WANT MY MONEY BACK

So I was handed over by finchoice to this debt collectors, the account handed it happened that I have already settled it with another debt collector agent. But because I didn't realize at the time So I paid them as per our arrangement, after I realized it's the same account as the one I have already settled I sent an enquiry notifying them its already been settled therefore I shouldn't have paid anything to them, it's been 3months now I haven't got a proper feedback to refund my money paid even though I have sent all the evidence to them including settlement letter, statement, proof of payment, to this day nothing has been done!!