Book 3 of the Seishun Buta Yarou series.

SOMETIMES MORE IS JUST CRAZY!
Just before summer break, Sakuta encounters Shouko Makinohara, a girl in junior high with the same name and face as his first love. Of course, that should be impossible, since she would be in college by now. Then, as if one mystery wasn't bad enough, Adolescence Syndrome strikes again. His impassive friend from the science club, Rio, discovers she has a doppelgänger. One of the Rios is forced out of her house and winds up staying at Sakuta's apartment. Definitely not the girl he expected to find himself living with...And when Mai insists she's going to sleep over, too, Sakuta's summer vacation descends into pure chaos.

(Source: Yen Press)