There's loads of unoriginal products, take Coke and Pepsi for example. There's loads of music that are very similar, like tons of pop music, even covers of the same songs. There are plenty of games that do the same thing. There are movies with the same plot, maybe like spider man for example. Same goes for books and everything else. But whether it's books, food, movies, songs, whatever, even though there are similarities, they all have something that is different to the other. It's called Unique Selling Point. It's what makes them different to all their competitions.

It can be a secret ingredient in a food or the way its cooked, it could be a different writing technique with books or poems/stories. With cover songs it could be the different voice or tone. Just because someone has had the idea of writing about X, doesn't mean you can't change it or improve it. So don't worry about oh someone's already written about that. That's not important. What is important is that it has your own spin or unique value to it that makes it different that other people can appreciate or enjoy