I now realise I've wasted money as I've done a lot of return journeys coming back straight away just to do the line, mostly extensions to the Overground network as they opened. I made sure I touched out then touched back in straight away. Presumably there's nothing stopping someone going touching in at Waterloo, go from Waterloo to Surbiton then back to Waterloo then to Vauxhall then touch out as long as it is within the maximum journey time for Zones 1 to 6. You would have to take care that you touched out at a different station than you touched in though, so going from Waterloo to Surbiton and back would be a no-no.