Why Is Cloning Bad?

I haven't figured out exactly why human cloning is a bad thing that should be opposed. It intuitively feels wrong, but that's a pretty poor standard to go by. In nature, human clones are all over the place - we just call them twins. But even if someone wants to clone themselves as a way to have children, why not? It doesn't seem inherently different from in vitro fertilization and most people accept that.

Someone please help me out on this one. Why should I be opposed to human cloning?