In Robert Louis Stevenson's novel, The Strange Case of Dr. Jekyll and Mr. Hyde, "duality" is one of the major themes of this story. The plot of the story allows us to envision the dual nature of Dr. Jekyll, the upstanding doctor, and Mr. Hyde, the violent alter-ego of Jekyll. The doctor releases the evil Hyde by ingesting a potion that triggers a change. According to Wikipedia.com:
This story represents a concept...of the inner conflict of humanity's sense of good and evil. In particular [Dr. Jekyll and Mr. Hyde] has been interpreted as an examination of the duality of human nature (that good and evil exists in all), and that the failure to accept this tension (to accept the evil or shadow side) results in the evil being projected onto others...
In other words, humans, arguably, have two sides: the good and the evil. This article goes on to state that if one does not recognize the presence of a darker side, that a person's moral balance will be thrown off, resulting in "evil being projected onto others."
eNotes.com identifies another major theme as "identity:"
Dr. Jekyll disturbs the natural order of the universe because throughout his life he struggles to accept the dual nature of his identity. He determines that all of us are plagued with this duality: "With every day, and for both sides of my intelligence, the moral and the intellectual, I thus drew steadily nearer to that truth, by whose partial discovery I have been doomed ... that man is not truly one, but truly two..."
Instead of leaning toward the good in himself, Jekyll tries to balance the two: this is the battle of good and evil, which is yet another theme. The duality of man is created by the struggle between doing what one believes is right or good, and fighting off the temptation to do that which one believes, through society and/or religion, to be evil or wrong. In trying to balance the two, Jekyll allows Hyde to get the upper hand. In giving in his evil nature through Mr. Hyde, the doctor realizes what he is doing, but also finds himself enjoying his "freedom" as Hyde (another theme).
Many of the themes in the novel point to the duality of human nature; freedom, good vs. evil, and identity all are visited in the struggle between these two opposing sides.
G. B. Stern...argues that the novel is "a symbolic portrayal of the dual nature of man, with the moral inverted: not to impress us by the victory of good over evil, but to warn us of the strength and ultimate triumph of evil over good once sin is suffered to enter human habitation."
Additionally, the concept of "duality" is seen in Victorian society at the turn of the twentieth century. Utilitarianism has found its way into mainstream society. It promotes the idea: "the greatest happiness for the greatest number..." This was also known as Benthamism—if it feels good for many, it's all right.
Contrary to the social climate of the time was the Protestant Evangelicalism movement which provided guidelines for behavior.
...the Evangelical's anxious eye was forever fixed upon the 'eternal microscope' which searched for every moral blemish...
Society and religion pull in opposite directions, demonstrating the struggle of the Victorian psyche for a balance within a state of "duality." The critical factor would apparently be the wisdom of resisting evil and leaning toward the good. If one is foolish enough to give in to evil, even believing he/she has the upper-hand, evil will successfully overwhelming good.