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 approaches her, claiming to be an old childhood friend. Maki has cared for Yori since he was a boy, but now he's a married man–married to a woman who looks exactly like Yori. This apparently doesn't stop him from chasing down Yori who desperately tries to push him away. What kind of future could this love (?) have?

(Source: the manga habit)