Some examples:
Yume de Aetera - past love/first love
Spoiler (mouse over to view)
Nagisa's first love, Minato comes back into her life and while she's in love with Fuguno, she almosts marries Minato.
Maison Ikkoku
Spoiler (mouse over to view)
Kyoko falls in love with Godai pretty early on, but she's hung up on her dead husband for a good bit of the story. Even when she's with Godai, it shows her feelings for her dead love still linger as Godai finds her crying over his items and the kicker is... IT'S A WEEK BEFORE THEIR WEDDING.