The plot sounds a bit like the ending to an Outer Limits episode called "The Joining". What I get is that a new element lets the protagonist create duplicates quickly. What I don't get is why they have different memories and skills. Where do those different experiences come from?

Because of the relations and shared experiences I guess, as explained.
What the premices and starting plot remind me is the 2009 movie Moon from Duncan Jones, so much.
And in this way I think they do not "repricate", I guess the "vehicule" is full of clones. (which are already grown up, but blank pages)

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Apparently not.

https://www.pcgamer.com/frostpunk-creators-dive-into-mind-bending-sci-fi-with-the-alters-a-deep-space-game-of-survival/

a rare element called Rapidium that enables the creation of alternative versions of organic matter.

The Alters are not clones: Each one is Jan at heart, but effectively a version of him that made different choices over the course of his life. Those choices led to different outcomes: Each Alter has a unique set of skills and traits, but also different attitudes, beliefs, and personalities.

So they're version of him that made different choices over the course of a life they never lived. The explanation of what the element does fails to explain why the duplicates have memories of things that never happened.

Well physically they ARE clones - some of them even comment on how they feel their bodies are different compared to how they expect them to be in their minds (and they have effectively been "implanted" with memories, skills, and experiences from the hypothetical "life path" as calculated by the quantum computer for that path).

The reason they have memories of things that never happened is because the quantum computer, as part of its calculations, has managed to factor in many things and can somehow (almost magically) predict what would have happened had this particular life path been chosen.

It's never explained how the quantum computer on a space craft, designed for interstellar navigation, manages to have access to what would amount to a vast amount of data about the lives of many more individuals than are supposedly recorded as life paths within it.

Or, more importantly, if the quantum computer can conjure up alternate realities and implant them into clones, where those realities include technology not currently in this reality (but capable of being in this reality as proven by the scientist), that why these QCs aren't just used to solve the Earth's problems by harnessing them as the ultimate AIs.

But it makes for an interesting story.