The thing is, the “duplicate Earth” gimmick is completely unnecessary to the story; in fact, it’s never mentioned again after the pre-credit teaser. It was just a cheap hook to keep viewers tuned in by ending the teaser on a “WTF??” note.I always liked "Miri" - the "another Earth" thing was just one of those "Gosh, wow, cool!" little skiffy frissons; I was already buying the notion of a spaceship full of Navy boys zipping around the cosmos at FTL speeds, after all.
Wellll...
As I understand it, the Alternate Earth idea was one of Gene's gimmicks to save production cost by filming in backlots and easy to get to locations while renting easily pre-existing props. So having it be another Earth in the teaser was a nod to those in the audience who might ask "why does their alien planet look just like Staten Island?" or wherever. Plus it's a way to say "Look at what might happen here if we don't shape up," to television audiences of 1960's sensibilities.
--Alex