A subspace divergence field, also known as a "spatial scission," is a phenomenon which has the capability to duplicating particles of matter. This phenomenon was first observed by quantum theorists at Kent State University in the 24th century. In this experiment, the scientists were able to duplicate a single particle of matter using a divergence of subspace fields, although the duplicate atoms could not occupy the same point in spacetime for an extended period of time before the quantum cohesion began to break down, causing the atoms to experience mutual annihilation. Although the experiment proved successful in duplicating normal matter, it failed when the scientists attempted to duplicate antimatter.
In 2372, the USS Voyager encountered such a divergence field in the Delta Quadrant, when it entered a plasma drift in an attempt to escape a Vidiian ship. This resulted in the existence of two duplicate Voyagers existing in the same point in spacetime, whose crews were initially unaware of each other due to both ships existing slightly out of phase with each other. Because the antimatter in Voyager's warp core was not duplicated, both ships experienced what they believed to be an unexplained drain on their antimatter supply, as both ships shared the same source of energy. Both crews came up with the idea of infusing the warp core with repeated proton bursts to keep their engines running. However, one Voyager began emitting the bursts before the other, causing serious damage to the other ship.
The two Voyager crews eventually became aware of each other's existence through a spatial rift which linked the two ships. They initially attempted to merge the two ships by recreating the divergence field and then depolarizing it by emitting a massive resonance pulse from their deflector dishes. The attempt failed, however, when the ships were thrown further out of phase with one another.
Eventually, the Vidiians found and attacked the undamaged Voyager, but remained ignorant of the other's existence. Deciding to destroy her ship to save the other Voyager, the Captain Janeway of that ship set hers to self-destruct, and sent Harry Kim and the newborn Naomi Wildman (whose counterparts had both been killed during the damage caused by the proton bursts) through the spatial rift to replace their deceased counterparts on the other Voyager. The Vidiians boarded the ship and took control, but both they and their ship was destroyed when Voyager exploded, leaving the other one intact, albeit with severe damage to repair. (VOY: "Deadlock")