Customer service says some customers are being double-charged. Our transaction table records the customer, merchant, transaction time and amount. For example, customer 101 swiped 250 dollars at the same supermarket at 10:02 and again at 10:04, then 80 dollars at 10:30. How would you use SQL to flag suspected duplicate charges and estimate the dollar amount involved?

客服反映有客户被重复扣款。交易表记录了客户、商户、交易时间和金额，比如客户101在同一家超市10点02分和10点04分各刷了250美元，10点30分又刷了80美元。你怎么用SQL找出疑似重复扣款，并估算涉及的金额？