Reason for this is because it happens to best of the brightest minds in fiction who had an idea for a story and someone called them on plagiarizing another one's work. Nothing says it than the old-fashioned "What a rip-off" from a fan with an eye for being critical. So if you have any interest in being a writer, you'll have to face plagiarism accusations at one point or another.
In fact, it happened to me once. When I joined an online writing group, I had a novel-in-progress receiving critiques, then one of them came down on me for ripping off the likes of an established author and a little mean comment came after that. I told the person that it's akin to what he knows so well, and he apologized. *Recalling what Peter David mentioned about the topic, he said it happens to him all the time. When he was writing Supergirl, fans called him on ripping off Joss Whedon's Buffy The Vampire Slayer. A character he created was revised to be a mirror of Whedon's Spike, especially when the character harbored feelings for the titular heroine. The character's creation predated Spike's creation by a year, and his being enamored of the book's blonde heroine predated by three years. Despite David coming up with his own ideas, no came after Whedon for biting off him.
Everything has been done before. In comics, movies, and books. Most likely what the other person came up with before will be an inspiration for the next project. But it's what they do with the inspiration that makes it their own. Translation: Ain't nothing new under the sun. Let's consider the following examples:
- Green Arrow-blended mixture of Batman and Robin Hood, with trick arrows instead of a utility belt.
- Swamp Thing-DC Comics' answer to Marvel Comics' Man-Thing.
- The Atom-Inspired by Richard Matheson's The Incredible Shrinking Man.
- Wonder Woman-Greek myths meets flag waving patriotism and the Lasso of Truth (That's the creator's idea. Don't ask.)
In my humble opinion, the line between inspiration and plagiarism is blurred every once in a while. It's important to separate the two things so everything doesn't get all out of order. On the one hand, it all comes with the territory.
*Source: Writing for Comics & Graphic Novels with Peter David, pg. 172.