UTAKATA: SUMMER PAIR OF EARLY WINTER

It's now winter in Kamakura. When Satsuki tells her friends that she'd seen Manatsu, Ichika at first doesn't believe it. But on the day before her 15th birthday, Manatsu's mirror fragment inside Ichika's desk drawer mysteriously disappears. That evening, a young girl calling herself "Mafuyu, Manatsu's identical twin" appears before Ichika with a mischievous smirk on her face. (Source: AniDB) — Description from AniList community (anilist.co).