So if Martha does go back in time I've sort of been assuming that she has to avoid her past self a bit.
But she has the TARDIS key-perception filter thing, which is basically a personal cloaking device as explained by Who-style psibabble. She could easily be in the background of every scene of the series so far{*}, and they could argue that the perception filter was working on the audience too. Hmmmm ...
{*} assuming she has full use of the Vortex manipulator -- otherwise, just the "present day" eps.
