Oh, Those Hanazono Twins, Volume 1 (Oh, Those Hanazono Twins #1-7)

Tatara Tani is so average it hurts - the only thing he has going for him is that his childhood friends are ultra-popular idol Yuriko Hanazono and her fashionable twin, Rinko Hanazono! They're both his classmates, and his long-standing relationship with them makes others seethe. But he thinks his fortunes might be turning around when Yuriko invites him to an amusement park on what probably isn't a date, seeing as she's explicitly NOT allowed to date as an idol. Even though Yuriko carefully disguises herself as Rinko, a photo of the two of them is discovered by their classmates... So he thinks he's hit the jackpot when Yuriko asks him to the amusement park with her, even though idols aren't allowed to date. She dresses up as Rinko to fool the world, but when her ruse is discovered by the school, it's up to the REAL Rinko to save her sister's reputation... by dating Tatara herself? This crazy rom-com of errors begins!