Have you ever wondered how you can create a unique book when it's eerily similar to another book that's already been written? Good question.
First, ignore the similar elements between your book and any other, and envision your story in your mind. Describe it to yourself. Fine-tune it. Flesh out the weak spots and trim the dead weight. Own it! Then, push the original story as far from your mind as possible. After all, there's a reason for the phrase, "There's nothing new under the sun." That applies to books as well as life in general. Get used to the fact that your book, no matter how unique you think it is, will be "similar" to another book or two or ten thousand in one way or another. So?
But, if you're still concerned someone may compare your work to a previous tome, emphasize the differences. After all, you're the author; you can write whatever you want. Be more detailed. Place your book in a different part of the world. Populate it with different characters, different places, different descriptive narrative, and different dialogue. Give your characters unique ethnic backgrounds. Set the story in a different time period. Use your own literary voice, of course, and not the other author's. (Which, I would hope, you're beyond temptation from doing anyway.)
For instance, you're writing a book about the Civil War and how your main southern belle falls in love with her cousin's fiancé but marries a scallywag who abandons her to her beloved Tara in the end. Oops. Let's revisit that one a bit.
You're writing a book about the Civil War, and your main northern belle (or Texan or Floridian or Vermonter) falls in love with her sister's fiancé but marries a former boyfriend, so she'll be better off financially than her sister. But, her new husband gets killed fighting in the War, and her brother-in-law dies of cholera, forcing the two widowed siblings to join forces to build a new life for themselves from scratch. And, no, not at Tara!
Admittedly, that second version is no Gone with the Wind, but it's similar. So what? Are you getting the point here? As long as you're not copying the other book word-for-word, using the same settings, and creating identical or even similar characters and descriptions, go for it, and let the chips fall where they may.
But, will that make your book different enough from the one that's already written? It will if you don't copy. Period. As in anything. No matter how great the temptation. Similar books aren't identical books and, thus, don't infringe upon one another. That's true legally as well as artistically.
Legally, while you can't protect concepts, ideas, notions, storylines, plots, thoughts, lists, names, film and book titles, common expressions, and about a million other things from use by others, you can copyright exact original phraseology and word-for-word passages unique to your work except for those that are in common usage.
So, you can copyright these sentences from your book: "Michael looked up, scanned the horizon, and leaned back on his rifle barrel before wiping the sweat from his brow and cursing softly. 'War is hell.'"
But, you can't copyright: "War is hell."
The first example is unique to your book, so no one can use it without first paraphrasing. The second example is commonplace, so anyone can use it without fear of repercussion.
But, to return to your question: How do you make your book different enough artistically from an existing book? By seeing it as your own story in your mind before describing it on paper without copying anything from the previous book, including even an obviously similar structure or plot. Then you won't have to worry about it.
In differentiating your book, keep asking yourself, What if this and What if that? And, of course, listening to your answers.
Got that? Good. End of story, end of fears, end of potential social or legal repercussions, end of question.
See?
Simple.
Next?