one and the same

Từ điển Oxford Advanced Learner 8th

▪ I. ˌone and the ˈsame idiom
used for emphasis to mean ‘the same’
•I never realized Ruth Rendell and Barbara Vine were one and the same (= the same person using two different names).
Main entry: ↑one_idiom
 
▪ II. ˌone and the ˈsame idiom
the same person or thing
• It turns out that her aunt and my cousin are one and the same.
Main entry: ↑same_idiom