Alipay once failed to pay, but the mobile phone received a debit message.

I suggest you look at Alipay's transaction records first. Do you have two payment records for the second item? If yes, there is a button to apply for a refund at the back.

If not, check whether there is any balance in Alipay. If not, are you sure your bank card has been debited? Call Taobao to ask.