Do not worry. I suggest you

to contact with the TOEFL中心 tell tell them that they double charge you and after they find out, they will give your credit back. Also even for the worse case, you always can contact your bank and dispute this charge at any time. But one thing you have to do is that you must follow up with the TOEFL中心 and make sure you do see your money back to your account after they promise they will give you credit back.