Client pays for bank's error

I changed the date of a stop order, with the result that the bank paid the amount twice. They have admitted their mistake but have refused to reimburse me until the beneficiary repays the money. I cannot make them understand that they should repay me and then reclaim the money because this was their mistake (incompetence????) not mine. Yet they have the nerve to say they don't want to lose my business! And I am a private banking client. I can only guess how badly they treat client with \ordinary"retail banking products!"""