[救救孩子] 有多个段落，如何提取出在每个段落中都出现过的 字/词/句/段落 ？

有五个段落：

para1 = "this is para one. I am cat. I am 10 years old. I like fish"
para2 = "this is para two. I am dog. my age is 12. I cn swim"
para3 = "this is para three. I am cat. I am 9 years. I like rat"
para4 = "this is para four. I am rat. my age is secret. I hate cat"
para5 = "this is para five. I am dog. I am 10 years old. I like fish"

希望提取得到如下结果：

this is para
I am
I

爸爸们，咋整？或者有没有现成的好用的 diff 工具能让我构造一条命令去执行系统调用，然后接收它的输出呜呜呜