Now I wouldn't dare get into this debate as I know sweet f-all about Shakespeare. But wasn't the genius behind his plays the fact that they were pretty much stories people knew already, just with his slant on them?
Or perhaps I'm not explaining that right. It's just something that I heard. It's not that Shakespeare plagiarised someone else's story, he just took well known tales and re-worked them. (kinda like Bram Stoker took the legend of vampires, and created Dracula. Still his creation, but he just took inspiration from mythology)