one by one 一个个地

He went through the items on a list one by one. 他逐个处理单子上的项目。

Students went out of the classroom one by one as soon as the bell rang. 下课铃一响，学生们一个个从教室里走出来。 

in one 合为一体

He’s President, Treasurer and Secretary in one. 他身兼会长、司库和秘书三职。

It's a book of English phonetics，grammar and usage in one. 这是一本英语语音、语法和习惯用语结合为一体的书。

one too many 多余的，不需要的

I found myself one too many among them. 我发现自己在他们当中是个多余的人。

one in a thousand 优秀的人（或事物）

My brother is a one in a thousand. 我的兄弟是位优秀人物。