Subspace doubling effect
Kira briefly duplicated by the subspace doubling effect
A subspace doubling effect was a phenomenon that could be triggered by a release of energy under particular conditions, resulting in the creation of an exact quantum duplicate.
In 2373, Major Kira Nerys was struck by an energy discharge when the USS Defiant passed through the energy barrier around the planet Gaia, which caused a subspace doubling effect that briefly duplicated her. Later, Yedrin Dax proposed modifying the Defiant to amplify the effect when it passed back through the barrier, so as to create a duplicate of the ship that would be thrown back in time and become the source of the planet's populace. However, Lieutenant Commander Jadzia Dax subsequently discovered the Yedrin had falsified the sensor logs showing the success of his plan, and that he never intended for the Defiant to escape the planet in any sense. (DS9: "Children of Time")