Peter Capaldi made his Doctor Who debut, not with the attack eyebrows cameo in The Day of the Doctor but rather, as Caecilus in the 2008 episode The Fires of Pompeii. He then went on to play John Frobisher in Torchwood: Children of Earth. Now this isn't really a problem, after all Colin Baker had played a different character before becoming the Sixth Doctor, and countless other actors and actresses have played multiple roles in the show with no mention about it, but Steven Moffie-Monster has said it will be addressed:
"I remember Russell told me he had a big old plan as to why there were two Peter Capaldis in the Who universe, one in Pompeii and one in Torchwood. When I cast Peter, [Russell] got in touch to say how pleased he was, I said 'Okay, what was your theory and does it still work?' and he said 'Yes it does, here it is'"And that quote from him is a good 18 months+ old and it's not been addressed yet. We've also had no proper hints that it's going to be anytime soon. Until now.
In a free iTunes podcast called 'Meet The Cast' Jenna Coleman and Peter Capaldi discuss the upcoming series of Doctor Who, and when the subject of a possible return for Caecilus came up here's what Capaldi had to say:
"With regards to my Roman alter-ego, there will be a… he may appear again."A quickly-stop-myself-saying-something-I-shouldn't-have moment there methinks, don't you?
Later on Capaldi further teases:
"[Caecilus] will certainly be invoked."So, what is the secret of the multiple Capaldis? What is Russell 'Tickle Me' Davies' plan? Will we find out in series 9?
All these questions and more may be answered from September 19th.