You might have heard that Rowling is being sued again over alleged plagiarism. The case itself looks, from what I've read so far, incredibly shaky and, via Boing Boing, there's an interesting post from Making Light about it. Teresa observes: "People who aren’t accustomed to having a lot of ideas of their own have a very poor grasp of the odds that others might independently come up with the same ideas."
There's been a lot of posting about films and books that have similar themes and events. This is, however, quite normal. (Neil Gaiman wrote an articulate post about this and other related issues.) I understand why some readers feel frustrated by similarities to other texts or what they consider to be derivative work, but I don't feel it myself. I do not weary of multiple tellings of Cinderella and I enjoy a story that is aware of the storytelling occurring around it. Those sly references to other tales entertain me. It means the author knows their stories. Different authors playing with the same general ideas, that provides the vitality of storytelling.