What is the "Twin Ciel" theory? Given the highly positive response to my last question: Who stole my candy?, there seems to be a lot of speculation around this "Twin Ciel" theory or the idea of there being 2 Ciels. 

So with that in mind, I am posting a follow-up question to that to ask for the full explanation of the theory and what this means for the series. 

Can anyone shed some light, links, etc?
So the twin Ciel theory has been floating around the fandom for a few years but has recently gained much more evidence in the last few chapters.

"The Double-Ciel Theory: A Kuroshitsuji Mystery" on Blogspot is the most in-depth explanation I could find, though it doesn't contain any recent evidence. To sum up the theory, some believe that Ciel


   is not the actual Ciel Phantomhive, but the younger more sickly twin of the real Ciel who was sacrificed.


In addition to the evidenced in that blog post, the most recent chapters show


   Agni looking at pictures of Ciel's family and saying "It can't be," then running off to protect Soma. He finds Soma with a gun at his head, and the person fires but misses and shoots Soma's hand. Soma then says "Why?" Now flash forward and "twin" Ciel and Sebastian come home to find the scene before them. Soma sees "twin" Ciel and hits him. Ciel also sees a bit of the picture Agni was looking at and says "it can't be," and then asks Sebastian "you wouldn't lie to me would you?" Which brings up the fact that Sebastian has never called "twin" Ciel, "Ciel"; only "young master".


These are by far the most compelling evidence so far the author has pushed towards supporting the twin theory.