Risk wants to flag transactions that are unusually large for that specific customer: anything above 2 times the customer's average transaction amount. Customer 101 has three transactions of 100, 200 and 900; customer 102 has 50, 60 and 70. How would you write this in pandas, and which transactions get flagged?

风控想找出对客户本人来说不寻常的大额交易：金额超过这个客户平均交易额2倍就标记。客户101有100、200、900三笔，客户102有50、60、70三笔。用pandas你怎么写？哪些交易会被标记？