Book 4 of the Seishun Buta Yarou series.

DOUBLE TROUBLE?!

​Sakuta is eager to finally have a chance to spend time with Mai again, but instead, he's greeted by a stranger who looks just like her. Apparently, Adolescence Syndrome is paying Mai another visit and has forced her to swap bodies with Nodoka Toyohama, a flashy up-and-coming idol...who also happens to be Mai's half sister! With no resolution in sight, each sister must pretend to be the other and go about their respective lives as best they can. Trouble is, it seems like Nodoka is preoccupied with something besides walking in Mai's shoes...

(Source: Yen Press)