The theme of duality is not confined only to the characters of the novella; Stevenson presents a Victorian London divided between strictly defined 'reputable' areas which are in close proximity to areas of poverty
The merging of two words 'combined' and 'mingled' to emphasise how it is impossible for humans to be only good or only evil, there will always be aspects of both in everyone's personality
The most obvious form of duality within the text, representing the conflict between Dr Jekyll's innermost desires and his outward presentation of himself
The observation that Mr Hyde "had nothing human" evokes fear and hatred in a contemporary reader who would have been terrified of the idea of 'devolution'
Stevenson uses this to show that the duality of human nature cannot be isolated, e.g. "Even as good shone upon the countenance of the one, evil was written broadly and plainly on the face of the other"
"[The] man was a fellow that nobody could have to do with, a really damnable man; and the person that drew the cheque is the very pink of proprieties, celebrated too, and one of your fellows who do what they call good"
This too, was myself: 'One of Jekyll's closing remarks, showing his awareness of the duality in human nature and his admittance of his commitment to "a profound duplicity of life"'
Stevenson's description of London heightens the sense of duality within the text, with alternating descriptions of the city having "an air of invitation" and buildings being "sinister"
The duality of good vs evil in Hyde's maid is similar to the conflict between Jekyll (one's desired outermost expression of morals to society) and Hyde (one's innate repressed desires, that exceed the bounds of society)
The reader's associations with the colour "black" would cause them to be suspicious of Mr Enfield who appears to be hiding something under the guise of darkness