In identifying the presence of a doppelgänger in Truman Capote's "Miriam," we must define what a "doppelgänger" is. The eNotes definition is:
In fiction, folklore, and popular culture, a doppelganger—is a tangible double of a living person that typically represents evil.
In German—the language "doppelgänger" is from—the word generally refers to a person's "double" or "look-alike." It is the mood of the story that gives the reader a clearer sense of which definition applies in this story. Capote portrays this double as a thief in the form of a child—taking Mrs. Miller's sens of self. Evil is present when Mrs. Miriam Miller sees Miriam (her doppelgänger) at her apartment late one evening.
The doppelgänger is very similar perhaps to a younger Mrs. Miller. What first makes the child seem suspicious is her old-fashioned dress. Miriam is surprised also that the child has never been to the movies, and that she is walking around without the supervision of a parent. Miriam Miller makes special note of her very adult-looking eyes:
...but her eyes; they were hazel, steady, lacking any childlike quality whatsoever...
Mrs. Miller goes into the movie and forgets the child. However, late one night there is a ringing of her doorbell. Miriam is puzzled and perhaps alarmed—the bell rings unceasingly, hinting at impatience. This moment foreshadows a serious change in the older woman's life because Miriam the child is standing there demanding entry. The child's persistence is at the very least rude, and at most, extremely aggressive in nature.
I thought you’d never answer, but I kept my finger on the button; I knew you were home.
Other details lend themselves to the presence of an evil doppelgänger. The child is intrusive and demanding. She argues when Mrs. Miller says it's late; she insists she be let in, and Mrs. Miller does not stop her. Another piece of foreshadowing alludes to the child's identity as Miriam Miller's "double" when the youngster speaks about "imitations," and how she dislikes them. Mrs. Miller's double "hates" imitations—egocentric as she is, will the child dislike Mrs. Miller, too—seeing her as the imitation?
"I like your place," she said. "I like the rug, blue’s my favorite color." She touched a paper rose in a vase on the coffee table. "Imitation," she commented wanly. "How sad. Aren’t imitations sad?"
Young Miriam knows things she should not know: where Miriam lives, for example. Tommy, the canary, responds to the child's presence even though it is evening and he is quiet after dark. Then Mrs. Miller discovers her guest in her bedroom, looking at her jewelry. Young Miriam demands that Mrs. Miller give her a cameo pin. Miriam futilely resists, telling the girl it was a gift from her husband.
"But it’s beautiful and I want it," said Miriam. "Give it to me."
Mrs. Miller can't resist young Miriam's request; it shows how very alone she is. Soon the child leaves. When Mrs. Miller next goes out, she shops, buying things almost against her will:
...a series of unaccountable purchases had begun, as if by prearranged plan: a plan of which she had not the least knowledge or control.
These are things the child wants. The pervading sense of evil comes from Miriam's dwindling sense of self as the child takes—until Miriam feels as if she is disappearing. The child is not harmless as a child might seem, and Miriam seems to be fading away.