(originally published October 1839 in The Gift: A Christmas and New Year’s Present for 1840)
A rich boy named William Wilson goes to school and finds that another boy who looks and dresses just like him, and who has the same name as him, also attends the school. At first the narrator and his lookalike get along, but eventually animosity sprouts. After a particularly spooky incident involving the lookalike, the narrator leaves the school. But the narrator cannot escape the other William Wilson, who continues to show up at various points in the narrator's life. Eventually the narrator kills the other William Wilson, only to find out that that other William Wilson was someone he didn't expect.
Summary written by Taylor Colegrove, May 2011. Title links to the full text of the story.