As a trope of science fiction there is a gulf between the fantastical idea of ‘clones’ and the mundane reality of the actual science of cloning. As of yet, actual human cloning has not taken place but primates have been succesfully cloned, specifically two crab-eating macaques called Zhong Zhong and Hua Hua. Part of the gulf in concept is illustrated by the commercial application of cloning techniques to pets. While cats and dogs can be cloned to create a genetically close animal to an existing pet, basic aspects of a cat or dogs identity including their pattern of colouring ae not solely determined by their genes. A cloned pet is not going to be identical, even at a superficial level.
So while the term ‘clone’ is what is used, actual cloning does not get at the concept which is more about duplication or near duplication. Creating another copy of a person is the essence of the science-fiction concept. Duplication of genes is just a handy hook on to which the idea can be hung. Practically we have always known that monozygotic twins are not literally identical even at a superficial level and certainly not at the level of character or personality.
So plot wise how do people get duplicated?
- Biological cloning. As discussed above this intrinsically doesn’t work because genes are only part of the picture. However, with enough hand waving and magic technology examples like the clone-troopers from Star Wars add in speeded up growth and education to create lots of near identical people. The horror of the Star Wars clones (never really explored in terms of the appalling aspects of the idea) is not creating duplicates of people but the idea of mass-manufacturing people. There is a under-explored view of industrialisation, as well as the dehumanisation of the military’s need for essentially fungible people to serve.
- Teleportation. Typically this is more of an unwanted side-effect of the teleporter-as-fax-machine concept. With the transmit-recreate model of teleportation there is a side-effect of possible exact duplicates being created, resulting in awkward questions of identity for Commander Riker or Kamala in Think Like a Dinosaur. There’s a fascinating bit of logical implication here that results in two quite different science fiction tropes being intimately connected. Any duplication process that can create a new version of a person (including their personality and memories as well as their body) implies a method of teleportation (transmit the information to a remote location and recreate the person). Any method of teleportation that works by capturing a person as a set of information likewise implies a method of duplicating people.
- Print people. Mur Lafferty’s Six Wakes, imagines a future where biological structures (including functioning brains) can be essentially printed by a kind of 3D printer that uses a stock substance. Taking a step away, and thinking about duplicating a person, then all the bits of a brain need to be there but it hardly matters if the actual cells contain the same cellular structure or genes just so long as they work the same way. Put another away, the resolution of the copy does not need to be so fine as to match genetics. Indeed genetics may actually be unhelpful.
- Robot people. When we think about it, if we want to duplicate ourselves (because we have become mad with power or seek immortality or so we can go to work & go to the movies at the same time) what we actually want is a being that thinks like us and looks like us. Does it matter if actually are duplicate is some sort of silicone layer over a titanium frame? Not at all, indeed that may even be a bonus. Biology is over-rated.
- Non-embodied duplicates. Of course to create a robot version of a person implies that it is possible to create a digital simulation of a person’s mind and personality. In that circumstance, do you even need a physical duplicate! A virtual reality duplicate may be more than good enough.
- Mirror-universe twin. Let’s get back to physically identical duplicates. If parallel universes exist then parallel versions of you exist. Here the not-quite-the-same aspects are a plot-feature. Evil versions of you sporting a goatee but still somehow like you are likely to crop up as soon as you start hopping around universes.
- Time-travel. Technically this is just the one version of you until you cause the time-line to splinter at which point the extra version becomes a parallel-universe twin. Even so, time travel affords a way of having more than one you in place at the same time with a simple (if mind bending) explanation of time travel.
- Pigeon-hole principle. If you have a universe that is big enough (e.g. infinite) and which has enough people in it (e.g. effectively infinite) then the number of combinations of possible features of a person ends up being less than the number of people. In that case, there will be some people who are just exactly the same for NO REASON AT ALL other than the universe’s lack of imagination.
- Reincarnation. Actual reincarnation beliefs are focused on the rebirth of an individual soul, although accounts may point to shared memories between the past and present versions. Fictional reincarnations provide a fantastical way for a person to be a duplicate of a long dead double. This is invariably bad news and tied up with all sorts of questions of destiny.
- Cosmic coincidence. This is effectively the same as the pigeon-hole principle but only assumes the two people happen to look exactly alike and does not require the same personality. The coincidence of looks results in a natural doppelgänger and leads to Prisoner of Zenda like shenanigans. You can’t make this just happen though (unless you use a technique above) so it doesn’t really count as a way of duplicating somebody.
- Surgery and hypnosis. We are now sailing into soap opera plot twists. Maybe X just looked a bit like Y and then an evil mastermind used cosmetic surgery and mind control to make them look more like Y and convince them that they are Y!
- Just bluff. In 1854 Roger Tichborne, heir to the baronetcy of Tichborne, went missing while travelling in South America. Believed to have drowned in a shipwreck, his mother nonetheless never gave up hope. In 1865 a bankrupt butcher from the inland town of Wagga-Wagga in Australia was encouraged by a lawyer to claim that he was, in fact, the missing Roger Tichborne. Thus began the long running saga of the Tichborne Claimant. Facially only vaguely similar and otherwise quite different looking, the question remains open whether this was a lost person found or a very blatant attempt to create a second Roger Tichborne simply by sheer assertion.