I was my understanding that a customer should receive notice of the sum to be taken prior to any direct debit payment, it appears that this doesn't apply to BT.
On 8/11/14 I received an 8 day notice as usual, on 15/11/14 I received notice of a refund, on 16/11/14 I received a credit note.
On 17/11/14 the amount referred to in the 8 day notice was debited from my account, on 24/11/14 the amount referred to in the 8 day notice minus the credit note was debited from my account.
Put simply, say the notice was for £100 and the credit was for £10, I paid £100 on 17/11/14 and then paid £90 on 24/11/14, a total of £190 instead of £90.
In addition I have been trying without success since July to get a charge removed from my account for a fictitous number, hence the credit note(s).
I did telephone customer services about this and was left " on hold ".
A contract or agreement with BT is worth nothing, it's no wonder people are leaving when this type of thing happens.
You need to be phoning billing, rather than the generic helpline.
Arguably, the first £100 is "correct" because they told you they were going to take the money, while the second was incorrect.
There is the Direct Debit Guarantee, which means you can go to your bank and insist they refund the money. But I wouldn't do that without talking to BT billing first, or you'll leave them even more confused than they are at the moment.