OK, we all know Doctor 12 won't be PC's Doctor Who debut, having been Frobisher in Torchwood & the Roman marble merchant, Lobus in The Fires of Pompeii. I read a while back that RTD had had an idea for a plot device that would explain why Doctor 12 looked the same as the other two characters PC has played, & that SM was actually going to use in in series 8.

But we don't know what the plot device is, anyone care to speculate?

I really hope there's no reference to the these characters looking similar. It's only because the same actor played them, there's absolutely no need to link them within the storyline.
After all, no links were ever made between Maxil and the Sixth Doctor, or between all the various characters in various stories played by, say, Bernard Horsfall, or Philip Madoc or countless other actors over the years.
Incidentally Peter Capaldi played Lucius Caecilius in the Fires Of Pompeii. Not sure who Lobus is...

Well we at least know that it will indeed be addressed/explained in some fashion

From radio times article:

"We are aware that Peter Capaldi’s played a part in Doctor Who before and we’re not going to ignore the fact," Moffat told Nerd3

http://www.radiotimes.com/news/2013-...er-appearances

most of the examples people give of when an actor appearing twice wasn't explained are from classic who when there wasn't so much technology around and people not watching episodes back as much (if at all).

As far as I can remember every time it has occured in new who, such as martha/adeola, gwen/gwyneth, and of course multiple clara's there has alway's been an explanation of some sort as to the resemblance, so as it stands it's almost the rule that they always explain such things in new who. That is why I'm glad that Moffat has stated he isn't going to ignore it. Shows respect for the loyal audience.

Relatively easily done with more peripheral characters, with the Doctor himself, not so easy, unless we're talking Chameleon Arches of course......

"Lucius Caecilius was the Doctor all along, he just didn't know it then!"

Caecilius est in horto.

That would make the make the Frobisher's ending in Torchwood much darker, even if that is possible.

I suspect it may be that the Doctor remembered Lucius and decided to look like him. Nothing more. After all, remember Romana in Destiny Of The Daleks when she changed into a copy of Astra saying it looked nice on the Princess.

It's not going to be anything more than that. It won't be.

I think just choosing the face would be a bit of a cop out tbh, but this one hadn't occurred to me - I like it.....

Has it's own set of problems though - like did he have a 'fob sundial' & can you cross your own timeline if you don't know you're a timelord & hidden by a chameleon arch?

Could different aspects of him be scattered across time like Clara or Scaroth?