If, in fact, the parallels are weak and not indicative of similar sources, then it shouldn't be incorrect to seek many parallels with other books.
I think we should clarify between the themes and plots. Similarities between themes I could dismiss easily, similarities between plots are harder to dismiss.
Let's look at some of the most prolific writing today... the paperbacks. Western paperbacks have been lampooned because of their similar themes. Every western plot starts with the hero (similar descriptions
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However, it is easy to tell when one novelist has borrowed/been inspired by the plot from another. Some regency era romance books borrow from Jane Austen and it is not just the theme that is similar, but also those bits and pieces that clue you in. I can provide two regency era novels, one written in the 1940's by Geogette Heyer and one written later in the 1980's that was a watered down version of her book. Not hard to find the parallels. Name of the characters were different, name of the town different, and the style was less polished, but they were the same plot. Tough to argue that they were just thematic element that were the same.
So....plot the same? Or simply similar themes...What I have bee reading of the posts leads to the conclusion that the plots are awfully similar.