Let me begin to answer your question as to how these two characters (Rodion Romanovitch Raskolnikov and Arkady Ivanovitch Svidrigailov) are extraordinary (especially in regard to duality). In short, both of these characters are often considered by Russian scholars to be aspects of Fyodor Dostoyevsky himself as sort of his two alter egos. Even further, Arkady Ivanovitch Svidrigailov is usually portrayed as a kind of dual Rodion Romanovitch Raskolnikov and as an example of what our protagonist could become if his instincts aren’t tamed.
In regards to Rodion Romanovitch Raskolnikov, he is extraordinary because of his connection to Fyodor Dostoyevsky in regards to obsession (and psychosis) having to do with both crime and punishment. Rodion Romanovitch Raskolnikov is quite smart. He is so smart, in fact, that he thinks he can outsmart the Russian government by performing two murders (rationalizing them by saying he will save his family much pain) and avoid the governmental consequences as well as the consequences of conscience. Rodion Romanovitch Raskolnikov isn’t thinking clearly, however, in that he is troubled by psychosis and poverty and hunger and sickness. After thinking up his plan, he kills two elderly women who are a burden on society. It is at this point that the suffering of Rodion Romanovitch Raskolnikov really begins and, through his penance, he realizes that he deeply loves his family, his community and his country. Life is truly worth living, even for two miserly old women such as the two he killed.
Next, in regards to Arkady Ivanovitch Svidrigailov, he is extraordinary because of his connection to Fyodor Dostoyevsky in regards to guilt as brought on by sexual urges. Further, he is often considered a double to Rodion Romanovitch Raskolnikov because the former is the horrible character that Raskolnikov could become if he doesn’t overcome and repent his evils. Arkady Ivanovitch Svidrigailov continually acts inappropriately towards Dunya. As a wealthy landowner, Arkady Ivanovitch Svidrigailov and his elderly wife employ Dunya as a governess. In the meantime Arkady Ivanovitch Svidrigailov becomes obsessed with Dunya and tries to seduce her multiple times. All Arkady Ivanovitch Svidrigailov wants is sensuality and sexuality. He has a reputation for seducing young girls twenty and thirty years younger than himself. He does end up giving (out of compassion or out of selfish convincing?) much-needed money to Dunya. Does this act make up for his behavior? No. He eventually puts a gun to his head and commits suicide, proving what our protagonist (Rodion Romanovitch Raskolnikov) could become if he doesn’t overcome his evil machinations.
In conclusion, you can see it is an interesting duality of character that connects both Rodion Romanovitch Raskolnikov and Arkady Ivanovitch Svidrigailov to Fyodor Dostoyevsky. Specifically, the duality refers to inner turmoil caused by guilt due to either murder or sexual misconduct.
As the previous answer provides a good overview of both characters, this answer will focus primarily on Raskolnikov.
The very name “Raskolnikov” comes from the Russian root “raskol,” meaning “schism” or “split,” which speaks to the duality of Raskolnikov’s character. One of the clearest illustrations of this double nature is in his quest to prove himself “extraordinary.” Perhaps Raskolnikov’s greatest inner conflict comes from the fact that he does not know whether he is a great man. Indeed, it is this very obsession that pushes him to murder the old pawnbroker.
Early on, Raskolnikov’s erratic behavior makes his motivations for murder difficult to understand. It appears as though he murders the pawnbroker to steal her money, however, he goes on to bury the goods he stole, rendering them useless. Only when we learn about Raskolnikov’s old article does it become clear that the murders were motivated by his utilitarian theory of the “extraordinary man.” In the article, Raskolnikov suggests that such an individual may have the moral right to transgress society’s laws and norms to fulfill some greater purpose or to increase overall good. A great man (which Raskolnikov imagines himself to be) would be able to commit and/ or get away with murder easily because his superior rationality would let him know that his actions are not, in fact, a crime.
Of course, in the actual commission of the murder, Raskolnikov’s utilitarian justification completely breaks down. He is utterly terrified and, in his panic, kills the innocent Lizaveta, proving that he could not be further from the cool, rational “Napoleon” that he imagines himself to be. Eventually, Raskolnikov reveals a second, alternate motive that is purely selfish, rather than utilitarian.
It wasn't to help my mother I did the murder—that's nonsense—I didn't do the murder to gain wealth and power and to become a benefactor of mankind. Nonsense! I simply did it; I did the murder for myself, for myself alone, and whether I became a benefactor to others, or spent my life like a spider catching men in my web and sucking the life out of men, I couldn't have cared at that moment. (Chapter IV)
Desperate to prove his superiority to the society he feels so estranged from, Raskolnikov murders simply to see if he is powerful enough to do it. Raskolnikov’s deteriorating mental state following the murders reflects the dissonance between his dual motivations for murder. He fails to recognize the inevitable dichotomy between the inherent altruism of utilitarianism and the ability to murder fellow human beings without guilt or remorse. In his quest to prove himself “extraordinary,” Raskolnikov ultimately proves his worst fear—that he isn’t a “great” man in any sense of the word.
Svidrigailov can be considered “dual” in that he acts as a foil to Raskolnikov; in some ways, he actually personifies Raskolnikov’s ideal of the “extraordinary man.” Unchecked by the morals or laws of society, Svidrigailov acts only to gratify himself and, as a result, commits terrible crimes, including the rape of a young girl. Unlike Raskolnikov, Svidrigailov has no dual motivation; he does things purely because he feels powerful enough to do them. He does not care for humanity or the greater good and, therefore, feels no remorse or guilt for his actions. Svidrigailov’s character shows us that Raskolnikov’s split nature, while having led to his mental devastation, is also what protects him from becoming wholly corrupted. Svidrigailov’s obvious moral decay serves as a glimpse into Raskolnikov’s future should he choose not to pursue spiritual and moral rehabilitation.