Since the concept of duality is very important in The Strange Case of Dr. Jekyll and Mr. Hyde, it is important that Stevenson carefully introduce it. In the beginning, we are treated to a description of Mr. Utterson.
MR. UTTERSON, THE LAWYER, was a man of a rugged countenance, that was never lighted by a smile; cold, scanty and embarrassed in discourse; backward in sentiment; lean, long, dusty, dreary, and yet somehow lovable. (ch 1, enotes pdf. p. 3)
Mr. Utterson is not the most sociable or fun person. He contrasts with his friend and walking companion, Mr. Enfield, “his distant kinsman, the well-known man about town” (p. 3). This is the first duality.
It was a nut to crack for many, what these two could see in each other, or what subject they could find in common. It was reported by those who encountered them in their Sunday walks, that they said nothing, looked singularly dull, and would hail with obvious relief the appearance of a friend. (p. 3)
By introducing us to two very different men, who are nonetheless friends, Stevenson begins a story of contrast and the dual nature of humanity.
The second duality introduced is the contrast between Dr. Jekyll, the upstanding citizen, and Mr. Hyde, who cruelly runs down a little girl. Mr. Utterson finds it hard to imagine that the two men would have anything in common.
“There is something more, if I could find a name for it. God bless me, the man seems hardly human! Something troglodytic, shall we say? Or can it be the old story of Dr. Fell? or is it the mere radiance of a foul soul that thus transpires through, and transfigures, its clay continent? The last, I think; for oh, my poor old Harry Jekyll, if ever I read Satan’s signature on a face, it is on that of your new friend.” (ch 2, p. 10)
Utterson assumes that Hyde must have something on Jekyll, because the two men are so different. Of course this foreshadows the fact that they are actually two sides of the same person.