USS Voyager (mimetic)

From Memory Alpha, the free Star Trek reference.

The mimetic USS Voyager was a copy of the USS Voyager created by the "Silver Blood" on a Demon-class planet in 2374. As part of the deal to let the real Voyager crew leave the planet, the crew allowed the Silver Blood to copy every one of them. The copies of the Voyager crew eventually lost their memories of their origins and left the planet, setting a course for the Alpha Quadrant.

On their journey, the mimetic crew invented a way to increase the speed of their engines and overtook the original Voyager. Several months later, the ship started to disintegrate, as did the members of the crew. They attempted to land on another Demon class planet but their attempt was thwarted when the aliens who controlled the planet refused to allow them to land and opened fire on the mimetic Voyager. They had no alternative other than to return to the Demon planet on which they were created. However, the ship completely disintegrated just as they were about to contact the real Voyager. (VOY: "Demon", "Course: Oblivion")

There has been much fan speculation on how many of the episodes between "Demon" and "Course: Oblivion" took place on the mimetic Voyager. Some episodes must have taken place aboard the real Voyager as the mimetic version of Tom Paris was still a lieutenant, while the real version was already demoted to ensign in "Thirty Days", a previous episode.