It's not a twin, it's more like his self conscience
His soul, self conscious, innocent side if you want to call it that, actually. If you noticed, the Ciel's double says "By sacrificing me" and then Madam Red says "by leaving your weak self behind.".
The twin theory is dead, let it go people. Madam Red was in the room when he was born, chapter 10 pg 16. No sibling. He is not an imposter (you can't recreate someones memories like your mother and aunt worry over you after an asthma attack (seen in the circus arc).).
Sorry, Anonymous, I didn't see your post, but you're sort of right. His conscious/soul/innocence that he sacrificed came to the rescue, along with his parents, and Madam Red, Joker, and Doll. Basically, Ciel is being questioned by his inner demons, and one of them is his soul/conscious, and Madam Red even confirms that when she calls the other his "weak self".
Why do people want to support this ridiculous theory still when it has been disproven many times over. Sheesh. I mean come on, last chapter pretty much gave us a clue that this was in Ciel's head, and once again this happened in Ciel's head.
Totally a twin. Would kick ass if he came back as an angel