The concept of duality is universal—it means that every aspect of life and the universe is influenced by opposing forces that exist in perfect harmony, such as the forces of good and evil. Stevenson argues that no one is purely good or absolutely evil and that humans are complex and multidimensional beings.
Good and evil exist within everyone—a good person is capable of doing evil things, and a bad person is capable of doing good deeds. The choices people make make every day, their behavior, and their actions are what defines them. However, that doesn't exclude the idea that the human soul is split, and that what people choose to do and who they choose to be depends on the internal conflict of good vs. evil or right vs. wrong. Furthermore, it is precisely this duality within us that make us human, which is why it is not only futile to try and separate the forces of good and evil, but it is also impossible, as one cannot exist without the other.
The attempt to separate good and evil is actually the exact reason why Dr. Jekyll can transform into Mr. Hyde. Like any other human, Dr. Jekyll sometimes struggles with immoral, strange, and sinful urges and desires, which is why he decides to experiment with some potions in the hopes of finding something that will completely detach him from his "evil" side—the one he desperately tries to repress. Naturally, he fails and only manages to bring his dark side to the surface and to allow it to coexist with his good side.
Dr. Jekyll despises his dark side and is convinced that the evil one is Mr. Hyde and not him, which is why he tries to "undo the evil done by Hyde":
Hence, although I had now two characters as well as two appearances, one was wholly evil, and the other was still the old Henry Jekyll, that incongruous compound of whose reformation and improvement I had already learned to despair.
… It was Hyde, after all, and Hyde alone, that was guilty. Jekyll was no worse; he woke again to his good qualities seemingly unimpaired; he would even make haste, where it was possible, to undo the evil done by Hyde. And thus his conscience slumbered.
Mr. Hyde is basically Mr. Jekyll's alter ego; he's the polar opposite of Dr. Jekyll, both physically and mentally. Or is he? Ironically, by making this clear distinction between them, Stevenson tries to tell the readers that Mr. Hyde and Dr. Jekyll are actually not different beings—they are one and the same, only they're opposites of each other, which is why when one dies, the other one dies with him. Stevenson uses this strange case of Dr. Jekyll and Mr. Hyde to showcase the duality of human nature—every person has their own Mr. Hyde, and every person has their own Mr. Jekyll. Good and evil exist in perfect harmony within the mind and soul of every human being and they cannot be divided.