Amane no Uta

One day, Amamiya, who is a school professor, comes home to find Naruse, a young man seeking shelter from the rain beneath his roof. He invites Naruse inside his home to dry off, but soon regrets his own actions. Usually, he would never have let a stranger inside, but the young man looked so much like his past love... Before he knew it, Amamiya had been seeing his past love within Naruse. A month later, Naruse appears before Amamiya once again, and asks to spend the night... (Source: MangaUpdates)

Included one-shots: Hatsukoi mo Naka, Kata Kutsushita no Otoko