overcharged

At the end of July I was overcharged, I called the call centre to complain and they informed me that I can wait for a refund which can take up to 40 days or I can leave the credit and they will take it off the following month so I decided to do that but guess what they double charged me again this month. I phoned again to query this and they tell me they will refund me within 40 days. What a disgrace, they make the mistake and I must wait up to 40 days .I have been with them over 20 years and this service is rather disgusting. Another option is they told me I can reverse the stop order at my expense' so I must pay for there mistake