Nine of Four wrote:
Kirk even says, "Not the Earth, another Earth........ Another Earth?". Yet that is never explained.
Nor is it ever mentioned again. The whole "duplicate Earth" thing was just a cheap hook to get viewers to stay tuned and watch the episode.
I like James Blish's adaptation which had the planet as actually a colony of human isolationists that dabbled in genetics gone horribly wrong. No mention of the planet being a duplicate Earth.