So, it sounds like her double-casting won't be part of some season-long mystery:

"It’s one of those very simple situations, like with Freema [Agyeman, who appeared in Army Of Ghosts before being cast as Martha Jones]: when you cast a great actor, you need a new companion...

...I was thinking, 'Should we go back to the 51st century, could we meet her again?' and then I suddenly went, 'Oh, let’s just cast her again. We’ve done that before. Lovely.' The relief that went around the entire production and the BBC, everyone went, 'Oh, great.' It was the most instant 'yes' you’ve ever seen...

...We had a great meeting. She was kind of amazed and delighted. Lovely. Good stuff ahead. Completely new character, again, a completely new story, and that’s a great new story that will run across eight episodes."