In filming the shot in which Doug #1 introduces Doug #3 to Doug #2, Doug #2 tosses a beer to Doug #1. Michael Keaton catching the beer (tossed by a body double) was shot first. Keaton tossing the beer (this time *to* a body double) was filmed later. Keaton's toss so perfectly matched the body double's original toss, that the planned special effect of digitally erasing the tossed beer can then adding in a digital beer can was not needed, thereby saving the production quite a bit of money. This was referred to by the special effects crew as "The Million Dollar Miracle Beer Can Toss".
According to director Harold Ramis, he wrote much of the script uncredited. A Writers Guild rule prevents directors from getting a writing credit on their film unless they contribute more than 50 percent. Ramis contributed 40 percent of the screenplay.
The trivia item below may give away important plot points.
Michael Keaton was originally considered for Bill Murray's role in Groundhog Day which was also directed by Harold Ramis and his love interest in that was Andie macdowell who also played his wife in this film multiplicity. But he turned it down.