Paul McCartney, upon composing the melody for "Yesterday", felt certain that it was too good to be original, and that he must have heard it before and just couldn't remember it:
McCartney composed the entire melody in a dream one night in his room at the Wimpole Street home of his then girlfriend Jane Asher and her family. Upon waking, he hurried to a piano and played the tune to avoid forgetting it. McCartney's initial concern was that he had subconsciously plagiarised someone else's work (known as cryptomnesia). As he put it, "For about a month I went round to people in the music business and asked them whether they had ever heard it before.
I am in no way comparing any story idea I've come up with to the creative work of McCartney, however, I do struggle with this same fear, especially in my preferred genre of science-fiction. As a consumer of science fiction across the ages (from Asimov, Phillip K. Dick, Twilight Zone, to Star Trek and modern works) it is increasingly hard to know / believe that I am thinking of anything truly original and that instead every idea is derivative at best and essentially unconcsious idea plagarism at worst.
I know there are some websites where you can search for themes and ideas
(TV Tropes being another one: I won't post the link lest anyone get lost for days there)
However, I am either failing to use these resources appropriately or they do not have precisely what I am looking for.
Do other aspirant authors have this nagging fear / issue? (Healthy fear of this is a good thing certainly - it keeps one honest.) In other words, how can you know / determine with reasonable good faith effort that your idea is original?