Are the transactions both posted transactions? Or is one pending and the other posted? Sometimes it takes a couple of days for the pending transaction to drop off the account and leave only the posted transaction.
Does your self serve account balance show a credit of $18? (HST$2.70 added to the credit card charge=$20.70) Wait a day or two and if both transactions are posted and you do not see the credit in your self-serve account you can contact customer support and ask for a refund which can take up to 30 days to process or a credit to your account to be used on your next renewal.
Please login to My Account and check the Payment History , does it show 2 payments?
Also, the 2 payments are on the same day or different one? Did you request plan change in the middle of the month
Or maybe the credit card showing 2 charges but one is a pending and one one is real charge?
Give us more details, like when are the 2 charges and how much