Page "Counter-Earth" Paragraph 10

It must also be noted that the Earth's orbit is not a circle but an ellipse, and in respect of Kepler's second law, a planet revolves faster when it is close to the star.

So if the Counter-Earth followed the Earth on the same orbit with half a year of delay, it would sometimes be visible from Earth.

Rather, to be hidden from Earth, the Counter-Earth would have an orbit symmetrical to Earth's, not sharing the second focus.