Dial direct

So I called dial direct to change my debit order from 15 the to the 23 Rd and to my surprise I was debited on the 22 of July the was no money and also on the 23 rd . And they called me and I asked them why was debited on the 22nd the was no money in my account and I was informed that I was supposed to be double debited the one for the 22nd it's for the month of July and the one on the 23rd was for the the month of August. To my surprise I have never skipped my premium and how can I be double debited whiles my account is up to date.