changing debit order date is a question of being billed double ????????????

DID YOU KNOW if you want to change your debit order date from the 1ste to the 10th you must pay double for that month. HOW STUPID!!!!!!!!!!!!!! If your salary date changes from the 1 st to another date discovery charges you for the 1ste and the 10th of that month. ALL I WANT THEM TO DO IS CHANGE THE DATE AND NOT DEBIT ME ON THE 1STE SO THEY ARE GOING TO AND ALSO ON THE 10TH-THE DATE I CHANGING IT TO!!!!!!!!!!!STUPID STUPID STUPID