If we want to find the double of the Earth, one of the first things to do is to find a telluric planet with a size similar to our planet. At least 17 billions in the Milky Way : it's the new estimate published by the American Astronomical Society on January 7th based on an analysis of data collected by the Kepler Space Telescope.
And a "sister" of the Earth must be in the habitable zone : it mustn't be neither too cold nor too hot to discover maybe, if the planet is telluric, life at least in the microscopic scale. It's one of the greatest dreams of an astronomer.
But among these 17 billion planets, is there one, two or more like our Earth ? Actually we know zero. But with Kepler Space Telescope, among 2 700 potential exoplanets, 4 planets have a size about 2 times Earth's and are in their own habitable zone.
The Milky Way is so big, too big for us. It's true that we'll don't find all the planets in our galaxy (only 850 found and cataloged), but find a planet with many similarities to the Earth would be a great discovery.