Hello;
I just re-subscribed to the game after cancelling my sub a couple days ago (I was going to play another game but changed my mind) and it charged my card after Battlenet said my next bill date was the 21st of August. Will I be charged twice or will the charge on the 21st be cancelled?
Thanks kindly
Are you certain that you successfully cancelled the subscription the other day? There shouldn’t have been a next billing date if it was. 
Do you have more than one game license?
If I remember correctly, the subscription now charges immediately because of the free items that come with it. The time would be added on to any time you currently have on your account. If your account said that your next billing date was August 21, then the new subscription would start at that date. You would not be billed additionally on August 21.
From what I can see, Dezu, your last subscription bill date was on 7/22 UTC, when you canceled a few days ago you still had time on the account so when you resubscribed it should have just added that option back. You won’t be billed again until 8/22 UTC, which should only be once during that period.
Thank you, much appreciated.