|You know it makes sense|
"Course: Oblivion" (VOY) is an unusual and significant episode. It raises deep and unsettling questions.
When the episode starts everything seems to be great, though anyone who had been watching the series would have had a slight sense of somehow having missed something. (When did Tom become a Lieutenant again, for example?) Voyager has a new improved warp drive that is zooming home, and Tom and B'Elanna are getting married. But B'Elanna falls sick and dies. The crew eventually realize the terrible truth. A while ago ("Demon") Voyager had landed on the Demon planet (class Y) where a silvery substance had duplicated the members of the crew, and the crew had sometimes wondered what happened to them. Now they know: it's them. Voyager itself was also duplicated, and the crew had somehow forgotten their origins. (Incidentally, the idea of the duplicated person who thinks they are the original was not in itself new in science fiction.) They've been travelling toward the Alpha Quadrant, apparently having Voyager-type adventures that we have never seen. Now radiation from the new warp drive is causing them to disintegrate: they had only considered its effects on humanoid lifeforms.
As the crew gradually die off and the ship decays, they make increasing desperate attempts to find a solution. They build a time capsule (of materials taken on from outside sources, so that it won't decay) to preserve their memory, but it's destroyed by a fault when launched. They find the real Voyager, which receives a distress call, but doesn't know what the ship is and finds nothing but small amounts of residue.
This is a downbeat episode, but there are many other downbeat episodes. What makes this one special is that it calls into question basic ideas and assumptions.
The first set of questions are about identity. What would it mean if you found that you aren't who you thought you were? This is a real question for some people, who discover things about their origins. An extreme example would be the children who were kidnapped by some dictatorships from their victims and adopted by the families of perpetrators. (Less radically, just discovering that your family didn't tell you the truth about adoption.) Also, there can be an identity crisis when something that has defined your life ceases to mean anything to you. In "Course: Oblivion" members of the (duplicated) crew respond in different ways. Captain Janeway only knows how to be Captain Janeway, and sees her mission as still being to get the crew "back" to Earth. Chakotay proposes going back to the Demon planet, as it is the best hope of survival, but at first Janeway resists this as it's the "wrong" way. Tom Paris, reverting to his earlier cynicism, feels their previous identity has lost validity, and questions why he should still obey the Captain. As he sees it, they are superfluous.
At another level, "Course: Oblivion" is about mortality, and about whether Star Trek has answers for such questions. In the Federation, people work to better themselves and others. In general, the vision is secular and this-worldly. You might fail, you might die, but there is a point to it all. But what if you are going to vanish, your achievements will be forgotten, and your life is not part of something larger? This is the fate that confronts the duplicate crew: as the title says, a journey to oblivion. Captain Janeway refuses to abandon the identity and purpose she has had: is this courageous, or a wilful blindness to the implications of what she now knows? Tom Paris continues going through the motions, though unsure why he should since he no longer sees any point to any of it. There is the hope that they can pass on a memory, but it is doomed. This voyage to oblivion is, if this world is all there is, the human condition. What response, if any, is possible? Do we side with Captain Janeway or Tom Paris? Alternatively, are they all asking the wrong questions? One might argue that whatever life they have lived, they have lived it: isn't that enough? Life as an end in itself? "Course: Oblivion" seems to feel that it isn't. It is raising the ultimate questions and leaving us with them.
Probably the only other time that Star Trek reaches this degree of despair is the brief scene in "Parallels" (TNG) where we glimpse an alternative reality where the Federation has lost the war with the Borg and the Enterprise is one of the last, hunted survivors. And even that doesn't quite have the sense of loss of personal value.
There are a number of other cases of people being duplicates in Star Trek, but they don't raise quite the same questions. In "Second Chances" (TNG) we learn that a transporter accident duplicated Riker some time ago, but this has only been discovered. Having lived apart, he isn't the same as the Riker on the Enterprise, and he is frustrated that he can't resume his life, because someone else has been living it. But he is just as much a valid person, and is able to re-enter human society.