Silver Blood

The "Silver Blood" was a biomimetic lifeform from a Y Class "demon" planet in the Delta Quadrant. 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, be it biological or technological.

Sentience
The Silver Blood was encountered by the crew of the USS Voyager in 2374. While attempting to mine the Silver Blood for deuterium, Tom Paris and Harry Kim came into contact with the substance and were duplicated down to their memories, Starfleet uniforms and combadges. Upon duplicating the Humans, the Silver Blood became sentient for the first time and quickly hungered for more. Holding 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 they could be copied. She agreed and Voyager was allowed to leave with a supply of deuterium. 

USS Voyager
By 2375, the Silver Blood had replicated Voyager itself, including its computer memory and programs, as well as The Doctor. By this time, the crew was able to breathe oxygen and had forgotten that they were copies of the original Voyager crew. Traveling across the Delta Quadrant while trying to return to Earth, which they thought was home, the crew experienced such encounters as the N'Kree attempting to conscript Voyager into their battle fleet or the Kmada attempting to sabotage their life-support systems with low-frequency theta radiation.

At some point during their travels, the ship had begun to utilize an enhanced warp drive, which would allow them to reach Earth three years after leaving the demon planet where the original Voyager would still have needed almost six decades to get there on their own. While the effect of this new warp core had no harmful side-effects on cellular life, it did however cause all people and objects made of the Silver Blood to eventually break down and die, striking first the engineering staff and subsequently spreading.

Having discovered their true nature after determining that the degradation only affected objects that had been on the ship before their visit to the demon planet – as well as testing the body of the recently-deceased B'Elanna Torres duplicate – the crew adjusted the environmental controls to mimic a class-Y atmosphere in the hope of slowing their degradation. Although the Janeway duplicate was still resolved to continue their remembered journey to Earth, some of the crew questioned her resolve to return to Earth; the Tom Paris duplicate was particularly bitter about it, immediately dismissing himself and B'Elanna as fake despite their recent marriage, calling Harry out on what would happen if he returned to Earth to find the original Kim present and wondering why they should even bother obeying the captain since she had no "real" rank. Aside from returning to a class-Y planet, The Doctor's only other suggestion for a cure was to find the original Voyager and take samples to stabilise their own condition, but Janeway dismissed that as impractical as they had no way of knowing where the original ship was in relation to them or if it even still existed.

An attempt to repair the ship by landing on another class-Y planet failed when they were attacked by a ship representing the Ord'mirit Mining Treaty, with Voyager too damaged to simply destroy the ship's weapons and Janeway reluctant to destroy another ship due to a misunderstanding. Despite her desire to get the crew back to Earth based on her own memories of their journey, the death of Chakotay during an argument forced Janeway to recognize that they couldn't continue in this manner, prompting her to order the ship to turn around so that they could reach the demon planet.

As time went on, the ship's degradation increasing as they progressed despite their use of Borg nanoprobes to try and stabilize the ship; by the time that they were five weeks from the demon planet, the holoemitters had disintegrated to the point that The Doctor's program was lost, and the senior staff were down to Janeway, Kim, Neelix and Seven of Nine, with Tom Paris incapacitated. Shortly before her death, Janeway suggested the creation of a log buoy from non-Silver Blood components detailing their history for others to find, but it was destroyed in the torpedo tube before it could be launched. With only ten hours of life-support left as they detected the approach of the original USS Voyager, the ship and remaining crew literally fell apart as Kim tried to slow them down by ordering the ejection of the warp core after all other controls were too damaged to continue; all hands were lost and the real Voyager never knew of the fate or even the existence of its counterpart, its discovery simply being a brief recording of an unknown tragedy.