"Then again, sometimes ideas simply stick with you and resurface unknowingly. I have no doubt that every author has inadvertently thought of an idea that was actually something they read about years ago"

This. Especially on topics where the work is based predominently on a concise/small core idea, such a tune in music. I once had musician friends psyched that they were writing a song based on a really cool riff, only to discover later that said riff was in a track they had heard a decade ago, and that had stuck in a corner of someone's head. They had copied by accident. And they found that out by accident, too, and might very well have ended up pushing a track that would have had them getting called plagiarists.