Well lots of things, let's be real, but in this specific case this is Sublimation by Isabel J. Kim.
The concept of the book is that it's an alternate universe where when a person crosses a border (it seems like this mostly means country borders although there are certain references to other types of borders), if they are not planning on coming back (consciously or not), they "instance," which is to say, they physically duplicate into 2 identical people, one of whom remains in the original country and the other of whom continues on to the destination. Both instances start with the same set of experiences/memories but from that point their experiences are entirely separate - they can't read each other's minds or anything. In this universe it is also possible for instances to "reintegrate" by coming into physical contact (the reintegrated person has all memories/experiences of both instances simultaneously).
It's about the immigrant experience really, specifically the Korean-American immigrant experience but immigration in general. And because that's its focus, I feel like in a lot of ways it doesn't delve completely into the full horror that this universe promises. The main characters are a woman who instanced at age 9 when her mother moved her from Korea to New Jersey (both instances of her) and a man who instanced in college when he was studying abroad in NYC and made a subconscious decision that he wasn't coming back when he headed from Korea to NYC after winter break (both instances of him). So, like, immigrant experience, alienation, who you left behind in your home country and your relationship with them, etc. etc. but both of these individuals are really pretty privileged people. There are background references to scary things like people instancing while being trafficked. But they aren't particularly front and center to the story being told because it's meant to be about the general immigrant experience, not a horror novel.
Anyway, the only Jew to appear thus far (228 out of 358 pages into a novel set partly in NYC) is in 1 very brief scene that one of the characters remembers. Here is what we know about her from that brief blip of an appearance: she is a startup founder for a tech company working on an advanced wearable that can prevent reintegration of instances in the event of physical contact for those who don't want to reintegrate, and she is "independently wealthy," and she is Jewish.
This character decided to get into this industry and create this product due to her genuinely horrifying family history related to instancing. Her grandparents just barely made it out of Europe before the Holocaust...but they instanced at the border. The instances who came out were fine. The instances who got left behind survived, barely. After the war they came to find the instances who escaped and wanted to reintegrate. This woman's grandfather (the one who escaped Europe) murdered his instance (the one who survived the camps) rather than reintegrate and have to live with the survivor instance's memories.
Again, this is deeply horrifying! These are the kind of implications of the universe I'm talking about when I say that the author doesn't fully delve into them. This is a really logical and meaningful reason for a person to want to create a product that could prevent reintegration - what if her non-survivor grandfather could've felt he was safely able to prevent an unwanted reintegration without literally murdering another version of himself that had already gone through hell?
...or so I think. The implication of the way it's written is very clearly that this startup founder is just in it for the money and using her sob story background for marketing or clout or something. This scene is a meeting between her and a character working for a company that wants to acquire her company, about which she is highly skeptical (and doesn't end up agreeing to it):
It had been an awkward meeting in a too-fancy coffee shop, which trailed into an awkward silence upon which you cast the weak conversational lifeboat of: Why an instancing start-up?
Hannah had punctured your lifeboat by telling you, with a vicious smile, about her grandparents, and the Holocaust. How they got out right before it was impossible to get out and how there was a second version of her grandparents, who had survived the camps, who had come and found them again when Hannah's grandparents were visiting family. And how her grandfather, the instance, had pushed his other self off a roof rather than live with the knowledge in his own brain.
You hadn't known what to say to that [...] To hell with people who use their family trauma as their conversational nuclear weapon.
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