I have a hard time believing there are that many planets that are "exactly" like Earth. It's one thing to say that a planet has an atmosphere that can support life. It's another thing to say that it's atmosphere has 78% nitrogen and 21% oxygen.
In dinosaur times, it is believed that the oxygen concentration on Earth was as much as 30% higher than today. Clearly the Earth was already a planet that could support life, but that doesn't mean we could breathe the air for long without appropriate gas masks. Yet in Star Trek, the air of "M class" planets is always identical to what we breathe.