Silver Blood

From Memory Alpha, the free Star Trek reference.

The "Silver Blood" was a biomimetic lifeform from a Y Class "demon" planet encountered by the crew of the USS Voyager in 2374. Appearing as metallic liquid in its natural state, this deuterium lifeform had the ability to change itself into that which it came into physical contact with. In this way the Silver Blood made copies of Tom Paris and Harry Kim, including their memories, Starfleet uniforms and combadges. These duplicates were altered to survive upon a Class Y world and could not survive in a Class M environment. Upon duplicating humans, the Silver Blood became sentient for the first time and quickly hungered for more. Holding the USS Voyager physically hostage on the surface of its homeworld, the Silver Blood copy of Kim negotiated to have Captain Kathryn Janeway give over samples of her crew's DNA so that it could be copied. (VOY: "Demon")

By 2375 the Silver Blood had replicated the USS Voyager itself, including computer memory and programs, as well as The Doctor. Also, by this time, the replicated crew were breathing oxygen (Or at least had adapted to cope with oxygen as well as the elements in a Class-Y atmosphere), had forgotten that they had come from the Silver Blood and were traveling across the Delta Quadrant trying to return to an Earth they thought was home. In this year the replicated crew activated an enhanced warp drive which had no harmful side-effects on cellular life. However, the enhancements caused all people and objects made of the Silver Blood to break down and die, first striking the engineering staff and subsequently spreading. Having discovered their true nature, they set course for the planet of their origin, but did not make it in time, and failed even to make a time capsule detailing their history for others to find. The ship literally fell apart in sight of the real USS Voyager; all hands were lost, and the real Voyager never knew of the fate of its counterpart. (VOY: "Course: Oblivion")