I realize that with all the serious fans on this board this topic has probably already been discussed, but I'm relatively new here and would like to bring it up if you don't mind.
We know of course why Naomi, Judith and Elizabeth all look alike - they are all Collins family members. However, how does one account for the fact that Mrs. Johnson looks like Abigail Collins? Or the fact that Willie looks so much like Carl Collins? Why would Julia Hoffman look like the Countess Natalie duPres?
My speculation:
Mrs. Johnson: I would hazzard a guess that a Collins male, perhaps Quentin in 1840, had an extra-marital affair which produced an illegitimate offspring, perhaps one he never even knew about. This offspring may have been the direct ancestor of Minerva Trask, which would explain her resemblance to the Collins family, and also why her daughter Charity looks like a Collins. Minerva's maternal line may also have been the ancestral line of Mrs. Johnson.
Willie Loomis: I would speculate that Pansy Faye wasn't the first "good time girl" that Carl Collins spent time with. He, too, may have had an illegitimate child, who in turn may have been the ancestor of Willie Loomis. (It doesn't seem as likely that Desmond would have had such an affair in 1840, considering his superior morals compared to Carl, but this is also possible.)
Julia Hoffman: We know that the Countess duPres was Andre's sister; so their parents were duPres. But perhaps the Countess had some family on her mother's side who weren't so well off as the duPres; they may have fallen in with a band of gypsies, resulting eventually in Magda, which explains her resemblance to the Countess. While it seems unlikely that the respectable and successful Dr. Hoffman was descended from gypsies, she too may have been descended from the Countess' maternal line, explaining the resemblance there.
The Collins family tree is certainly a tangled one. Have you any theories of your own about them?