Double Debiting

I changed the debit order date for my RCA as they said I can't cancel it. The consultant assured me that since it was less than two weeks before the date it would only be effective from the next month. 1st September the old debit was processed. This morning the debit order was put through again on the amended date. At least advise clients that if you change the debit order before it goes off it will be duplicated and go off on the old and the new date then I would have left it until a later date.