Ane no Kekkon

Iwatani Yori is a 39-year-old woman who leads a quiet life while working at the library. She thinks things like love and marriage are too troublesome. A handsome, but suspicious man appears before her, claiming to be an old childhood friend. While suspected to be a fraud, Maki has cared for Yori since he was a boy. But he's also a married man married to a woman that looks exactly like Yori. This apparently doesn't stop him from chasing down Yori who desperately tries to avoid him.