The title is a riff on the concept of a lone gunman being behind the Kennedy assassination.
When they are trying to lure out the Kennedy clone they sing "Happy Birthday", in a similar vein to what Marilyn Monroe did for president Kennedy. The two were having an affair at the time.
Grassy Noel's name is a reference to the much cited conspiracy theory that there was a shooter on the "Grassy knoll" during the Kennedy assassination.
Among the clones are: Elvis Presley, John F. Kennedy, Tupac, The Beatles, Stanley Kubrick, Abraham Lincoln, Avril Lavigne, Josef Stalin, Rasputin