Perceiving similarities and differences in art, life - and wit (successfully applying them in literature and in a joke is another matter):

Vladimir Nabokov –“Despair”

        “You’ll say next that all Chinamen are alike. You forget, my good man, that what the artist perceives is, primarily, the difference between things. It is the vulgar who note their resemblance. Haven’t we heard Lydia exclaim at the talkies: ‘Oo! Isn’t she just like our maid?’ ”
          “Ardy, dear, don’t try to be funny,” said Lydia.
        “But you must concede,” I went on, “that sometimes it is the resemblance that matters.”
        “When buying a second candlestick,” said Ardalion.

Sigmund Freud -  "Wit and Its Relation to the Unconscious" (1916).http://www.bartleby.com/279/1.html

"Since time immemorial a favorite definition of wit has been the ability to discover similarities in dissimilarities, i.e., to find hidden similarities. […] Vischer refutes this, however, by remarking that in some witticisms there is no question of comparison or the discovery of similarities." [He] "defines wit as the skill to combine with surprising quickness many ideas, which through inner content and connections are foreign to one another. K. Fischer then calls attention to the fact that in a large number of these witty judgments one does not find similarities, but contrasts; and Lipps further remarks that these definitions refer to the wit that the humorist possesses and not to the wit that he produces."

Brian Boyd “Vladimir Nabokov the Russian Years”:

“In his thesis ‘The Aesthetic Relations of Art to Reality’ Chernyshevsky advanced the claim that art is only an inferior imitation of a prior reality, material and obvious and commonplace. Fyodor proposes instead to reverse priorities, to show that life follows art.  He believes things cannot be understood only in material terms: those who really attend to life discover that the play of consciousness they must exercise to apprehend their world seems to correspond in some mysterious way to a force of conscious playfulness somehow concealed behind life. […] Fyodor…loves to accumulate, but he prefers the stray, disregarded detail to the accepted commonplace[…]  History for Fyodor will always surprise us. In retrospect, though, we can trace its themes – which will not be shared generalizations, but unique patterns of individuality. Life prizes the particular, allows things to develop as differently as they can, but somehow traces its patterns through the very differences between things, and leaves individual imaginations to perceive these differences and patterns in their own way.” (458)

Other examples related to resemblances between people wittily explored by V.Nabokov:

Despair: "...It even seems to me sometimes that my basic theme, the resemblance between two persons, has a profound allegorical meaning. This remarkable physical likeness probably appealed to me (subconsciously!) as the promise of that ideal sameness which is to unite people in the classless society of the future…" 

Pnin:  "Pnin and I had long since accepted the disturbing but seldom discussed fact that on any given college staff one could find not only a person who was uncommonly like one's dentist or the local postmaster, but also a person who had a twin within the same professional group, I know, indeed, of a case of triplets at a comparatively small college where, according to its sharp-eyed president, Frank Reade, the radix of the troika was, absurdly enough, myself; and I recall the late Olga Krotki once telling me that...  there were as many as six Pnins, besides the genuine and, to me, unique article. It should not be deemed surprising, therefore, that even Pnin...could not help becoming aware ... that a lanky, bespectacled old fellow... — a person whom Pnin knew as Professor Thomas Wynn, Head of the Ornithology Department, having once talked to him at some party about gay golden orioles, melancholy cuckoos, and other Russian countryside birds — was not always Professor Wynn. At times he graded, as it were, into somebody else, whom Pnin did not know by name but whom he classified, with a bright foreigner's fondness for puns as 'Twynn' (or, in Pninian, 'Tvin').

Pale Fire:  "Take my own case," continued [Shade]  "I have been said to resemble at least four people: Samuel Johnson; the lovingly reconstructed ancestor of man in the Exton Museum; and two local characters, one being the slapdash disheveled hag who ladles out the mash..."[...] "I would rather say," remarked Mr. Pardon — American History — "that she looks like Judge Goldsworth" ("One of us," interposed Shade inclining his head), "especially when he is real mad at the whole world after a good dinner."

Pale Fire: “Don’t you see [almost tugging at Shade’s lapel] the astounding similarity of features — of the upper part of the face, and the eyes, yes, the eyes, and the nose bridge?"

"Nay, sir" [said Shade, refolding a leg and slightly rolling in his armchair as wont to do when about to deliver a pronouncement] "there is no resemblance at all. I have seen the King in newsreels, and there is no resemblance. Resemblances are the shadows of differences. Different people see different similarities and similar differences."(CK,line 894)