If you see an error message when checking out, then it indicates that the payment didn't go through even though it shows as pending on your bank account.
Even when the payment is pending, the payment can still have been already declined. So, don't worry too much about multiple pending payments -- it doesn't necessarily mean that duplicate payments have been made. Please check your bank records in a few days, if multiple payments have been charged (instead of pending) on your card, please contact us at <email@example.com>.
There may not be anything wrong with your credit card. When you have multiple transactions very quickly, some banks become cautious; this is just their security measure.