Duplicate charges with no explanations

Charge was debited from bank account, description was " sale of forex". When this was queried nobody at the bank could say why this was done. When trying to report to ***** department they claim that the charge was for a global payment done 6 months prior, for which charges had already been debited. They cant explain how the new charge was calculated or why it was debited 6 months later, and after their charges had already been paid. Complaints department response was simply that this is a charge for the transaction done 6 months earlier (charges already paid at that time), no further explaining as to why they decided to add additional charges over and above the charges already taken at some random time in the future. Based on this , are they going to just keep taking random amounts for old transactions in perpetuity?