Unauthorised Debit Order after sending notice f resignation

I sent a notice of resignation to Discovery Health on the 25th of June notifying the scheme that I will be serving notice during the month of July because I am moving to a different scheme. Come the 1st of August, Discovery still went ahead and debited my account with my monthly premium. This means I have two active memberships which are not necessary.

Discovery, please pay the debit order amount back into my account because this has affected my finances adversely.