Imagine you meet your doppelganger of a different species. They look nothing like you, but your personalities are as close to identical as possible! -- Anon Guest
The universe is a fractal, as above, so below. Galaxies spiral into a supergalaxy. Stars spiral into galaxies, clouds of debris spiral into both stars and planets. Atmosphere spirals into weather... liquid drains into a spiral form. Around and around it goes... Is it no wonder, then, that there are patterns of personas?
They do not always look alike, of course. It's hard for a lizard to resemble a gigantic spider, for example. Many times, though, they occupy the same job. Many times, they have exactly the same mannerisms and patterns of speech. They have the same habits, the same quirks, the same... essence as another you might know. They may even have a similar name.
Jean-Paul Satre theorised that hell is other people. A. J. Rimmer knew without a doubt that hell is an eternity with yourself. Those are just two extremes that result from meeting your animus echo. Your... soul-doppelganger, if you will.