How Publishing Really Works is throwing an anti-plagiarism day today.
Plagiarism is easy to spot and prosecute when actual wording has been lifted from another work, but stealing characters, plot structure, and ideas is a gray and murky area.
Obviously some ideas, like "a kid who finds out he has special powers and has to go away to school to learn about them", are general enough that nobody's stealing from any particular source when they write yet another one of these. And there's no question literature is rife with repeated tropes and deliberate allusion. That's all ok.
But then there are the cases where there's really no question where the ideas came from... and no sense the author expected you to make the connection. I can think of a particular author's characters who have shown up more than once in other books... with no indication that it was "an homage" of any kind.
Over and over I get asked about the danger of writers' ideas being stolen at publishing houses, and I roll my eyes, because it's not a danger at all.
And because when it comes to stealing ideas, as heartbreaking as it is, the danger (though still not so common) is most likely to come from other writers.