Alternate Earth (Dead on Arrival)

Jo Grant was briefly sent to an Alternate Earth when the molecular adjuster on the the Doctor's TARDIS malfunctioned.

The planet was identical to that of N-Space, right down to its inhabitants. When Jo passed through to this reality, its inhabitants were unable to see her due to her existing on a different dimensional plane, but her presence caused the life force of her alternate counterpart of this reality to disappear, killing her, and causing Jo to believe that she herself had died after seeing this reality's Third Doctor visiting her open coffin at a church.

In the end, Jo's own Third Doctor managed to find his way through to this dimension (killing his own counterpart in the process) to retrieve her, while also preventing a potential invasion of that world by the Breelians. (COMIC: Dead on Arrival)