The "doppelganger" here is the speaker's own anxiety, imagined as a thin, fidgety girl who has followed her since childhood. She first appears under the table, whispering frightening stories: thunder is demons, a blister could be fatal, her mother will die. At ten, when the speaker realizes parents can lie, the double convinces her that nobody can be trusted and she must handle everything alone. Even the happier teenage years, with books, guitars and borrowed cool, include her.
The tone is wry and affectionate rather than bitter. Small details, like the furrowed brow, the picked scabs and the Hitchcock films, make the fear feel like a real, familiar person. In the last section the speaker has gained some control. She tells the double to shut up, and the evidence of a long life, a forty-year marriage and a grown child, proves the predictions wrong. Yet the ending is honest: the double retreats under the table but never leaves. The mood is quietly resigned, a bit tired, and oddly tender.