Final answer:
A doppelgänger is a character who is a physical or psychological double of another character. They can be used to explore themes of identity, duality, and self-discovery in literature.
Step-by-step explanation:
The question is referring to a literary device known as a 'doppelgänger.' In literature, a doppelgänger is a character who is a physical or psychological double of another character. In the given examples, the main character keeps seeing another main character in the same places he frequents, indicating the presence of a doppelgänger. An example of this can be seen in the quote from the text, 'He was a shadow, a phantom-a part of the encompassing unreality.' This suggests that the 'doppelgänger' character is an elusive and mysterious figure. The use of a doppelgänger can be a way for authors to explore themes of identity, duality, and self-discovery in their works.