Tempest Car Hire is not trustworthy

Tempest are claiming that they have duplicated a refund of deposit and have requested I pay it back. I have requested information from them on numerous occasions only to be supp**** with the date and amount that was refunded. The problem here is I rented cars on a weekly basis over a period of a couple of months through Tempest and often had to call to request my deposit a week or so after returning the vehicle. These refunds would be done adhoc sometimes two at a time as rentals overlapped. Is it amiss of me to ask for specific dates and refunds for all the rentals over that period to show proof of the duplicate. Not according to them. There legal representative says I must go through my bank statements and prove that there was not a duplicate payment otherwise there will be legal action. Very unprofessional of Tempest. On top of this I receive receive SMS's from the legal firm and daily phone calls threatening legal action. Tempest prove the duplicate payment and I will pay you back but if you are not willing to go to a bit of trouble well maybe legal action is your only respite.