You can say that again! Is an emphatic way of agreeing with something someone has just said
You can say that again是強烈地表示同意某人剛剛說過的話的方法。
附上例句讓你更了解:
A:Believe it or not.Ken won the lottery yesterday.What a lucky guy he was.
信不信由你,肯昨天贏了樂透,真是個走運的人。
B:You can say that again. Some people are born lucky.
我完全同意。有些人天生就是運氣好。