Warnings:
Wendy, I'm very sorry. Yes, it could be any other name; it's just bloody convenient to abuse a name, saves a lot of words.
Snuff, not necessarily, but the most likely reason for anyone to set up the world as suggested is to make that, if not justified then excused.
Absurd, yes, the premise hinges on a random act of magic; it's there exactly to create impossible situations that have to be addressed "realistically" then, resulting in unthinkable consequences. And yes, I go off on the very math of absurd, sorry.
***
Wendy, she was the prettiest sweetest smartest girl in the class, school, town, the entire universe. Tom was dumb dweeb who had a hopeless crush on her like so many others. One day he found courage to ask her out, for an upcoming event with dances and what, and was, of course, given a curve. But Wendy was too sweet to make it hard. She said something to the effect "sorry Tom, I'm going with Ryan, maybe next time." And Tom was delusional enough to think that the problem is she's taken. So, he thought, like, "if only Wendy could double up and I then could have my very own Wendy..."
Through whatever random act of pure magic his unreasonable wish was granted. Wendy doubled up. An exact copy of Wendy appeared in the world, likely naked as any newborn should be, and very horny (as that was the intention behind her creation), and most possibly at least a little disoriented.
How the encounter with this magical creation went for Tom we may or not learn, it's actually immaterial to the story. I'm mostly interested in the long term consequences of his foolishness.
What matters is, the spell was badly worded or the magic complied twisting it in horrible way as so often happens with those wishes. Particularly, it didn't stop with doubling Wendy only once.
Every time period of N another copy of Wendy is created, for every alive Wendy in existence at that moment. If it's every Friday, then next week we have four Wendies, and eight in a fortnight.
One may still think, cool, every horny guy in the class can have own Wendy. But in less than three months there's over thousand of them (2^10 = 1024) and that's when you start having a problem for sure, as there's a million in five months (2^20 = 1,048,576). Current population of humans on Earth is little over 2^33, Wendies doubled every week could exceed that in under nine months (alhough at that rate, there's likely a lot of natural loses before then). A year later? In theory, there would be approximately 4,503,599,630,000,000 Wendies! That's four and a half million billions. Let's dub that eventuality Wendycalipse as there's no way to sustain millions of billions of Wendies no matter how much we love them.
So, if the spell cannot be undone, or until the time a way is found of how to do that, the only way to prevent Wendycalipse is to control the population of Wendies the hard way. Not before the support systems would break at first locally. There will be starving people, and Wendies. Actually, the ultimate solution would be to kill all and every Wendy. But that won't gonna happen, even if that realization would eventually come. It's just too bloody convenient to have disposable teen girls around. Think about all the medical testing, organ transplants (for those lucky enough to be compatible with Wendies) and what not. The old simple slave labor... and of course all kinds of deviant desires. Wendies would be kept, but the first rule of keeping a Wendy is that you're obliged to kill all the waste Wendies that keep appearing in her vicinity.
One week for the doubling period is just an example, and it's probably still much too short for any chance to prevent Wendycalipse in time. Different values of N would lead to potentially quite different outcomes and stories. Small enough N and Wendies will suffocate each other in crowds appearing at doubling time much sooner than most realize. A large N, like one year may seem manageable conventionally, without intentional culling, for the first couple decades, leading to potentially very hard moral conundrums that become so much more pressing later, as after thirty-some years the Wendycalipse would still come swiftly. Even longer periods and we need to determine would the magical doubles age and if so, the determine the peak population of Wendies... but those would be millions, possibly billions of old lady wendies in that case.
Something like a month might work best, short enough for the horror of Wendycalipse to be readily apparent (it still exceeds the maximum breeding rate of rats (up to about ×1,250 per year, while 2^12 = 4,096)), but long enough to be managed (we do manage rats), and for the moral choices presented to be hard on most -- Wendies are people.
Or are they? They are created by what's undoubtedly some kind of magic, so all but original may live by different rules if it suits the story. There's a range of possibilities.
It's probable, even necessarily for the problems that magical Wendies need food like anyone, and some clothes for thermal insulation if not necessary cover, they should require resources. But millions and billions of Wendies would constitute a problem even if they don't need to eat, really, only want to.
Do magical Wendies age? The original being a teen, likely a late teen at first it may not be immediately apparent if they don't. A pretty girl can look almost the same for twenty years nowadays, with good care. If they don't, the inevitability of Wendycalipse is so much clearer, you can't just wait them out to die off from old age. The original would eventually, but some may hope that would break the spell and there for sure would be would be vigilantes after her, so it's not necessarily likely she survives that long. But if she does die young the copies may get freezed at that age. Alternatively, each copy age as normal, but all new appear as teenage, although it's probably much more interesting for each new Wendy to be double of a particular Wendy as she was moments before. That would immensely benefit those using Wendies for slaves, if they doubled with all skills and training. I would choose the copies being always teen to be explicit it's not the case, but that can be mixed as needed as well.
So, what stories can be told in such universe?
The creation story is interesting, and doesn't even necessarily need to be dark, those coming long time after.
Perhaps the moments when it becomes apparent Wendies need to be culled for the order of the world to be preserved is the most interesting, and lead to very different places.
Long after that there's lots of possibilities. Wendies might be established fixture as common slaves or otherwise, or be declared illegal and hunted down, surviving by people who hide them. There could be Wendy camps, enclaves, even states, or alternatively Wendy meat farms... because, why not? It's free meat, if you cull them fresh.
One could write about perils of keeping a wendy, or of problems of coming across a runaway one.
Could be a farce, pure slasher porn, or even serious parable about many things, not necessarily out in the face, or a hardcore morality study.
I just, I had this in my head for a while, and the context of the other thread about snuff stories gave me impulse to actually write this much down.