I want my money back!!

They handed my closed account to 2 collectors, I set an arrangement with one of them the other I never spoke to their agents. 30th of May they deducted the entire owed balance, on the 8th the other collector charged for the arrangement whereas the account is now fully paid off.They won't refund my money .when I speak to their customer service they don't even have working contact details for this DEBTIN collector, they fail to assure me they'll bring my money back and have their collector cancel the debit order. The worst experience ever