Double debit without consent

My debit orders are due to go off on the 1st of every month and for some reason you insist on debiting on the last working day of the month no matter how many times I request the change. Yesterday I have a double debit go off my account without my knowledge or consent. No one from your accounts department has contacted me with regards to this. According to my statements the amount went off at the end of last month and no indication was given that the amount was returned. I need this sorted out as soon as possible as your staff are incapable of assisting me