The double is neither the doppelgänger nor the difference that instantiates it. As an English language noun and verb, double is, semantically, value-free, quite unlike the German doppelgänger, whose English equivalent is an apparition or ghost of a living person. In that sense the double, as the idealized repetition of the same, arrests difference. The double is thus an index of the moment or "now" of eternity, the paradoxical repetition of non-repetition or, if you will, the repetition of a repetition that makes no difference. Thus, this impossibility—repetition of the same without any difference—is often, at the level of culture, the name of horror, of evil. Sophocles' Oedipus Tyrannus embodies this pollution: Jocasta is "the same" person as both mother and wife; Oedipus is both her son and husband. And while we might understand why incest appears to be a taboo among homo sapiens regardless of epoch or location, twins are an abomination only among some indigenous and pre-modern cultures. The myth of Remus and Romulus, for example, "explains" the ascent to grandeur and descent into madness that not only "writes" the history of the Roman Empire but also writes the history of all human civilizations, per Edward Gibbons, as an endless cycle of sui generis greatness and self-degradation. For Gibbons, every great civilization is thus marked as merely one example among other great civilizations, and so the proliferation of examples becomes the repetition of "the same." This non-differentiation of ascent and descent erases the subject/object and induction/deduction dyads (good/evil, particular/general, etc.) that inaugurate theology and philosophy insofar as these disciplines offer "faith" and "knowledge" as the only paths away from inevitable descent (respectively, damnation and superstition). That is, inasmuch as theology and philosophy point to one another as the obstacle to salvation (religiosity as mere superstition, speculation and knowledge as indices of pride), both are Siamese twins, two "heads" sharing one corpus, the double sign of horror and evil medicalized as abnormal, from the start.
Abnormality incarnate, the twin is a kind of genetic "disease." More generally, the twin is a synonym for disease, the mutation of an originary self into its benign doubles (e.g., bacteria or viral infections that are not life-threatening) or its doppelgängers (e.g., cancer). Put another way, is it possible to imagine, much less conceive of, a singular twin, a twin that is not already one half of twins? We may as well imagine love without hate, peace without war, health without disease, good without evil, life without death, etc. And yet, are not life or health, as mere examples here, precisely the denial of their others, their doubles or doppelgängers (and the choice of terms makes all the difference to an individual, a culture or a nation), as twins? In short, the "differences" between life and death, health and disease, are not binary oppositions, polarized positions, but "only" minute deferrals, slight differences, of and from homeostasis and sentience.
Of course, homeostasis and sentience are not simple concepts. If one—fauna, flora, culture, nation, etc.—is born dying, born obsolescent, born without a conscience, however conscious, then the grounds for artificial intelligence, to say nothing of the grounds for "knowledge" (and I'm thinking of Alan Turing's mathematical formulae for game theory, decision-trees, and computation in general), have always been in place, long before the technology was available to facilitate their realization. Advances in all the physical and human sciences—not under the rubric of progress so much as a fine-tuning of limitations and, thus, future possibilities—have complicated our concepts of communication, knowledge and the "nature" of what we too often prejudge as life and its alleged double (or doppelgänger), non-life.
However construed, the differences between twins and doppelgängers cannot be reduced to objects of knowledge. Only an entire network of contexts can organize those decisions that, by their very nature, differentiate. And insofar as mathematics, for example, has its own, and necessarily, idealized solutions to complex problems, it too, as a science, can serve either or both the twin and the doppelgänger. For example, Kylan Rice's meditations on anatomy and classification deploy mathematical systems that, by their differences (e.g., topography), consume and prepare their "objects" for consumption. Thus, as Rice notes, there is no "rounded" leaf because sentient existence—flora and fauna—is irreducible to the idealized angles, lines and shapes of description and analysis. Geometry is not geography, nor, per Alfred Korzybski, is geography the territory.
Historically, the "problem" of twins and doppelgängers has often been reduced to a matter of interiors and exteriors; philosophically, this is the bugaboo of "other minds." In Debra Di Blasi's work the doubling of the live/digital divide reproduces the extrinsic/intrinsic problem—a "real" apposite a finger, the latter which exteriorizes what is interiorized—or rather an interior constructed from the dance of fingers, Genetically, the twins/doppelgängers "divide" underwrites the nature/nurture debate, explored in its most classical forms in the dreamscapes of Heather Campbell: waters and architectures as the bookends of bio- and self-determinism even if the binary opposition is riven from the start.
Perhaps this begins without endings in the architectonics of languages, in, for example, the English/German problem of translation qua consumption. Allison Spence's work is initiated here as twins and doppelgängers. Insofar as one twin can kill off or absorb its other half, the facilitation or movement or transformation from twin to doppelgänger is mediated by consumption. This has been as much a concern of translation (how can one translate either syntax or semantics without bowdlerizing, if not completely digesting, the "original") as the grammar that "informs" any one language. These questions are posed in the various articulations of Maged Zaher, Paul Konecki and Nicholas Grider. Inasmuch as grammar and syntax mediate the "health" of a language, citation is always a potential threat; it can sometimes pose as the semantic twin or doppelgänger of a text. And if the condition for the possibility of language is repetition, the iteration of linguistic structures, then citationality is the articulation of difference "itself." It is fitting that Aaron Kunin brings together the problem of "disease" as a matter of citation. What is outside (and thus marked by quotation marks in American English) informs the structure and nature of an inside. We know what it means to be "healthy" only because of our prior understanding of what it means to be "sick." Ditto for an "original" and its "copy"—or twin—or, and, doppelgänger.