The piano motif throughout the film comes from the song 'Der Doppelgänger' by Franz Schubert; the words to this piece tell the tale of a man and his evil twin.
The first line of Yasmin Paige's character, Melanie, is "Idiot!", the title of another novel by Fyodor Dostoevsky, author of the novel upon which this film is based.
In the "Replicator court Marshall" extended scene, the central member of the court Marshall board is played by Laurence Fox, Son of James Fox who plays the Colonel.
Simon: I have all these things that I want to say to her, like... Like how I can tell she's a lonely person, even if other people can't. Cause I know what it feels like to be lost and lonely and invisible.
Simon (Jesse Eisenberg) desperately wants to be a Somebody instead of a Nobody, so he creates a "Somebody" who encompasses all the qualities he would wish for but would never dare manifest. When the Double/Phantasy gets out of hand it has to be gotten rid of, and so he takes drastic measures.