Do you remember the Arnold Schwarzenegger remake of The Parent Trap called The Sixth Day? Where Arnold played a pair of twins that were the result of a guy being cloned against his will?
That scenario has also been the basis of more than one Star Trek episode, only there the clones tend to wind up with opposite personalities of the original, even to the point of assuming outright Evil Twin qualities. It's a classic mindfuck plot, since the one asshole in the universe none of us want to contend with is ourselves.
For delusional victims of the syndrome of subjective doubles, that recycled old sci-fi plot becomes a stark and iron clad belief. Some people suffer it as another symptom of the above-mentioned Capgras delusion.
Sufferers of SSD maintain that they have a twin who looks exactly like them but has a completely different personality, presumably brandishing an evil twin goatee. Sometimes patients even allege there are multiple clones running around, or that their own personality has been copied and pasted into some stranger.