ON THE DOUBLE

Dave's President Bill Mitchell (Kevin Kline) isn't the only one with a ''twin.'' Turns out the Warner Bros. movie, which has grossed $47 million in its first five weeks, has a look-alike in the 1992 novel Throw the Rascals Out! Here's the Real McCoy, by political writer Otis Carney. While many details aren't the same, book and movie both include a chief of staff with a Machiavellian streak; a suddenly incapacitated President who's replaced by a naive double ignorant of the ins and outs of Washington; and a public charmed by the President's new vigor. Warner Bros. and director Ivan Reitman say they had not read the book. A spokeswoman for Dave screenwriter Gary Ross maintains he finished the script before Rascals was published. ''Believe me,'' says the spokeswoman, ''the screenplay didn't change appreciably afterward.'' Neither side is planning legal action, and Carney himself writes the coincidence off as a ''funny damn thing.''