Martha Jones (clone)

Martha Jones was cloned from the original Martha Jones in order to help the Sontarans to start a war with Earth. As her first job was to deactivate the missiles that UNIT sent to the Sontaran Ship. She then assisted the doctor (presumably unknown to him that she was a clone, but this isn't certain as he told her that she might as well have worn a T-shirt saying "clone"). And she tried to stop him from going to the room with the Cloning Pool. But he found out that she was a clone and woke the original Martha up. This broke the link between them and clone Martha died in front of Martha. (DW: The Sontaran Stratagem/The Poison Sky)