There have been not one, but two duplicate Kryptons, created to fool invading races. An odd tactic, but a recurring one, so it made sense to the Kryptonians.
Once, "many decades ago" Krypton was faced by an invading force, a "savage and terrible war with raiders from another galaxy" and Krypton's greatest scientists made an artificial planet, a shell that looked like the true Krypton and "blacked out" their own planet, when the invaders came. Jor-El wasn't shown, and presumably it was before his time.
The shell possessed artificial gravity, simulating Krypton's own, though it was a shell. The "blacking out" of Krypton was obviously more than just turning off their lights---perhaps some sort of field that totally made them blend with the space around it? When the invaders attacked the wrong Krypton, the true Krypton attacked and destroyed them. (All of a sudden, Fort Rozz looks pretty reasonable.) The extra-galactic aliens were revealed to be as humanoid as Kryptonians or earthlings...in fact, the few survivors stayed on the "alternate Krypton" and was initially mistaken by Superman as Kryptonians.
Perhaps Jor-El was inspired by the earlier fake Krypton, which had wandered out of Krypton's solar system (and would, decades later, wander into Earth's) because when a similar threat happened during his lifetime, he devised the atom-scan ray to make a model of Krypton from the meteorites in his solar system and even provided an artificial red sun much bigger than Krypton II, and Krypton II was inhabited by androids simulating Krypton's entire population.
Krypton II fell into its artifical red sun and was destroyed at the end of the story.
These two elaborate decoys help illustrate the cultural xenophobia of the Kryptonian people. Despite being supposedly descended from two space travellers stranded on Krypton, they had suffered disaster after disaster from aliens. The Vrangs enslaved the entire population for years. Virus X, their deadliest disease, was brought to Krypton by an astronaut from a "plague planet". Their capital city, Kandor, was taken from them by the mysterious Brainiac. It doesn't seem to be a racial trait--Superman shows no sign of it--but does seem to be an understandable cultural bias.
One suspects the Science Council's refusal to listen to Jor-El was partly influenced by their own xenophobia, that the idea of living among primitive aliens was repugnant.
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