Merging of Doppelgängers: The Cosmic Treadmill's Strange Power

The Cosmic Treadmill was a device invented by Barry “Flash” Allen powered by cosmic energy. This device enabled Flash to travel through time, with positive radiation drawing him into the future and negative radiation drawing him back into the past, coordinated by pulsations emanating from its cosmic ray-powered clock. 

During the Crisis on Infinite Earths, Barry along with fellow Justice Leaguers and members of the Justice Alliance of Earth-D constructed a larger version of the Threadmill. The became an interdimensional transport for millions of inhabitants of that world, which were being destroyed by the anti-matter wave destroying the Multiverse. Working with his counterpart Tanaka Rei, Barry created another treadmill, with the ability to merge doppelgängers!

Barry shared this invention with his nephew Wally "Kid Flash" West and his counterpart Jay "Flash" Garrick of Earth-Two. Jay later created his own version of the Threadmill, the Time Vortex. This mechanism similarly allowed for guided time travel, as well as fabricating period-specific clothing for individuals using it. Star-Spangled Kid suspected it manipulated cosmic energy like his  cosmic converter belt and Starman's cosmic rod.

While on Earth-Prime, Barry created another treadmill allowing him to return to Earth-One. Now realizing it bend time and space, upon Tanaka's suggestion they modified the Earth-D version. Those individuals of Earth-D travelling through its portal merged with their "genetic doppelgängers" on Earth-One. Of course, some like Barry and Tanaka weren't genetic counterparts, and so would've remained separate.

A similar occurence happened years earlier, when the Aquaman of Earth-One encountered the city of Necrus, which shifted from its native universe to another from time-to-time. On this occasion, the beam that brought Necrus to his world also struck the King of the Seas, causing a doppelgänger of his appear to be splitting off from him. In fact, this was the long lost Aquaman of Earth-Two, whom Arthur Curry's mind was connected with during this process. Together they defeated Mecrus' leader Mongo and his evil forces attempting to invade Atlantis and attack the surface world, although at the cost of the second Aquaman's life.

The device causing Necrus and the golden age Aquaman to shift from universe to universe was a "dimensional time reflex" machine created by a scientist from a doomed world in space. While merged, the Aquaman pulled into Earth-One's universe was mentally tied to that planet's native Aquaman, as would the Earth-D citizens with the Earth-One twins.

Even the transmatter cubes, created by the Justice League and Justice Society to travel to one another's worlds annually, was used to this effect. When Earth-One was invaded by the creature known as Krogg, who had resided in the Limbo realm between universes for centuries. the Kal-El and Kal-L used the device to become a composite Superman. Just as they doubled their own power, the two felt each other's minds slowly become one.

Something similar probably occured with the Freedom Fighters of Earth-Two, recruited by Uncle Sam to protect Earth-X. Each member may have merged with their Earth-X doppelgänger, which had near identical lives to their own (aside from not being superheroes themselves) and allowing them to carry on their wartime adventures chronicled in Quality Comics.

Perhaps the most widespread effect of this occuring was when the alien Creator2 tried to merge Earth-Two and Earth-One together using the android Red Tornado's dimension-travelling ability. On this occasion, all of the worlds' citizens met their doppelgängers of he other world for a brief period.

What effect this merging had on the minds and bodies of those of Earth-One who now gained the lives and experiences of their Earth-D doppelgängers in unknown.