Bob meets Elica, who's a dead ringer for his very-much-alive companion Alice... and then Elica dies, shaking Bob to his core and causing him to realize how much he cares for Alice.
Alternatively: After Alice's death, Elica enters his life, and simply being around someone who looks so much like his dead lover is such a painful reminder of all he's lost that he can barely stand to be in the same room as her, especially if he blames himself for Alice's death.
After Alice's death, Bob finds love with Zelda, who's as different from Alice as possible, because being with her helps distance him, if only briefly, from memories that have become too painful to bear.
After Alice's death, Bob falls in love with Carol, who looks nothing like Alice but has almost the exact same personality.
Subverted: After Alice dies, Bob meets Elica, but shows no interest in her or in starting a relationship with her.
Double Subverted: ...because, it is revealed, he has already started going out with Celia.
The artist has Only Six Faces in their repertoire, and Bob is instantly attracted to anyone with a face drawn like Alice's.
"We've replaced Bob's normal Alice with Janet. Let's watch to see if he notices."
Zig Zagged: Alice supposedly dies, so Bob finds a girl called Elica who looks exactly like her, but it turns out that Alice was really alive, except Bob has fallen in love with Elica for real, and then Elica dies, and Alice is going out with Rob...
Averted: Bob ends up getting with someone completely unlike Alice.
Enforced: "Oh no, we just killed off Bob's love interest! Let's make a character who looks exactly like her so the audience won't be too bummed out!"
Lampshaded: "Wow, a new girl who looks exactly like my deceased girlfriend, Alice! How's that for a coincidence?"
Invoked: After Alice's death, Bob seeks out a lookalike.
Deconstructed: Bob latches onto Elica purely because of the physical resemblance, only to grow disillusioned with all the minor and major differences between his lost love and her lookalike. Elica, for her part, is less than thrilled upon learning Bob was attracted to her because she looked like Alice.
Reconstructed: While Bob is originally attracted to Elica because she looks like Alice, he ends up falling in love with her for who she is, not who she resembles.
Played For Laughs:
Bob accumulates a whole harem of Alice-substitutes, and calls them all Alice.
Alternatively: Bob sees resemblances to Alice that aren't actually there.
Played For Drama: Bob accidentally refers to Elica as Alice during sex. That makes things rather awkward. The whole replacement issue creates issues for both of them.