Kelly Coyne at The Atlantic:
As a senior at Smith College, in 1955, Plath submitted her thesis on the doppelgänger—the concept of finding a mirror image, or a look-alike, in another human. The notion carries sinister connotations: In a 2014 piece on doppelgängers for The Atlantic, Alissa Wilkinson noted that “encountering your match has long been considered a harbinger of death.” Novels that participate in the doppelgänger tradition tend to illustrate the deadly struggle between protagonist and double. Just think of the creature who becomes a threat to Victor Frankenstein in Mary Shelley’s novel, or the fluctuation between Robert Louis Stevenson’s refined Dr. Jekyll and the violent Mr. Hyde.
On a trip to Plath’s archives at Smith, I was able to read her thesis, in which Plath described the double as being made up of “the evil or repressed characteristics of its master.”