Current story may involve a character suffering from Fregoli Delusion. If you haven't heard of this condition it's part of a whole class of delusional misidentification syndromes where the brain basically fails to associate a person, place or object correctly. If you look at some one you've known you're entire life and you think they are an imitation you may have a Capgras Delusion. You believe that you are already dead you may be experiencing Cotard's Delusion.
In the case of Fregoli, the believer will maintain that people he or she meets are all in fact the same person in a variety of disguises. The delusion is named after a turn-of-the-century quick change artist, Leopoldo Fregoli, who would astound audiences throughout Europe with rapid, seamless alterations of identity during the course of this stage shows. Fegoli Delusion, like the DMS's, are often caused by traumatic brain injuries, particularly those causing damage to the prefrontal lobe.
Fregoli Delusion is interesting to me because of the many narrative possibilities that disorder represents. First off, I want to say sincerely, that if you or if someone you care about suffers from this condition please don't take my ramblings on this topic to be anything more than the musings of a partially informed amateur genuinely curious about misfirings of the brain. I mean no disrespect.
The Philip K. Dick identity paranoia aspect of this, though, are truly amazing. Imagine, for a second, the life of someone with this delusion. People, maybe everyone, maybe just one subset of the population are actually all the same. Actors playing a variety of roles. It would be like living in Cloud Atlas (the movie) all the time; people you know are Tom Hanks keep putting on ridiculous gobs of make-up and pretending to be someone else. It would be living with a conspiracy so incompetent, its very cack-handed existence was a insult to your intelligence.
"Seriously, people, at least try to cover things up!"
Somewhat more sinister would be a person where the entire world was somehow occupied by a shape-shifting other, like the alien from John Carpenter's The Thing. Everyone you knew had already been assimilated, it just hadn't bothered with you yet. Or possibly, like the 70s version of "Invasion of the Body Snatchers" you might convince yourself that by acting 'the part' you might evade detection. Your life would become a very unpleasant movie.
From the literature I've been able to track down, the delusion can be more localized, however. The believer might identify only a handful of people who share the Fregoli identity. These quick-changers persecute the believer, stalking them. Sort of the faceless Men in Black at the core of every conspiracy story.