Eragon is Eragon, but he's also Eragon. Huh? Come again? The title of this book obviously is the name of our story's hero. (Although it's not his secret name—for more on that, see "Themes: Identity," but then rush right back here.) But our Eragon is not the first Eragon to come down the pike. He's actually named for another Eragon: an elf and the first Dragon Rider.
In this way, this seemingly super-simple title neatly sums up one of the central dilemmas that Eragon faces in this book: the burden of the Dragon Rider legacy. It's important to remember that Eragon did not go online to DragonRider.net and fill out an application to become one of these mythical heroes. Instead, he inherits this power with his name. In this way, the title both tells us who the book is about (Eragon), and also what the book is about (Eragon's quest to fulfill his legacy).
Pretty clever for a teenaged author, don't you think?