About Toby Fox's music, I don't think the point here is really plagiarism. He's not plagiarizing. The point is that practically ALL of his songs seem derivative of other compositions (usually just a single composition), and then again we have the idea of the "modder". It's as if he's modifying songs he likes.
Of course, if you analyze any musician, you're probably going to find a number of influences, but it's one thing to determine influences, and another thing to be able to line up all of a composer's work and show that there's a "sibling" song to that one.
That's not so common, I think. I may be wrong, however, perhaps it is indeed possible to take any composer and show that all of their songs have another song by a third party that influenced them, that sounds very similar.

Note: In my opinion, that doesn't make Toby Fox a fraud or anything like that. I think he's good at what he's doing. But I think he's a "modder" at heart.