poor customer service

I went to the garage on the 31st May when paying for my petroleum with my debit card the transaction declines which was not possible so we swiped again which is successful when I get home my notifications shows I was debited twice I contacted them they told me the fault is with my bank I than went to my bank who confirmed that engen did debit from my account I sent them my statement and they insist it's my bank. I waited 19days for them to refund my money which fault is with them, I am now so scared to use my debit card at any garage, the money they debited was for me to take my child to the doctor I couldn't coz they debited twice, when I asked them about compensation they told me its my banks fault, if it was my banks fault why did the refund come from engens bank. Disgusting service