In Doctor Who - Season 2 Episode 12 a character who looks identical to Martha is turned into a pseudo-cyber woman when a couple of ear pieces are fused to her brain. Later in the episode the ear piece is pulled out and a piece of what looks like her brain is clearly visible. It's stated that they can't be pulled out or that she'll die.
Then, just a few episodes later, she's a hospital worker and becomes the doctor's companion!
Is there any explanation for this?
We've already heard the doctor state that once someone dies it becomes a fixed event.