The reference work The Worlds of the Federation by Shane Johnson confirms the indigenous name of the planet as Earth, but the world was subsequently called Onlies by the Federation. The star it orbits had the designation UFC 347601. Not only the planet, but also the entire solar system was an exact duplicate of the Sol system. After its discovery, the Federation began to colonize the planet. Scientists explained the planets existence by the theory that at some point in the distant past the Sol system moved through a massive tear in the space-time continuum, resulting in replication in the subatomic level. (from the Memory Alpha Star Trek Wiki)