In Synecdoche, New York, there's a scene where Philip Seymour Hoffman checks the address list in an apartment building. One of the names on the list is "Capgras." Meanwhile, a few years back, when Charlie double-billed with the Coen Brothers in Carter Burwell's "Theater of the New Ear" "sound plays" project, and scheduling conflicts meant that the Coens were unable to stage their play in L.A., an unknown playwright named Francis Fregoli stepped up to the plate. Fregoli, it turned out, was Kaufman again, writing under a pseudonym.

There's a write-up on io9 which explains where those names come from. The article's called "Everyone Around You is An Impostor: Inside the Mind's Most Bizarre Delusions."

The first of the delusions, in which a person becomes convinced impostors have replaced their friends and family, is known as the Capgras delusion, named for the French psychiatrist Joseph Capgras who along with Jean Reboul-Lachaux first described the condition in 1923.

[...]

Leopoldo Fregoli was one of the greatest entertainers of the late nineteenth and early twentieth century. He perfected a style of performance known as quick-change, which is pretty much exactly what it sounds — he would switch costumes and characters during his stage shows so rapidly that it was suggested he actually required several other Fregolis for his act to be possible. And while that notion of hidden duplicates would fit right in with the delusions we've been discussing, that's not actually what won him his unlikely — and quite possibly unwanted — immortality in the annals of psychology. (Source)

More info at the link.