The Switch was written in 1978. Elmore figures he got the kidnap idea from the O Henry story, Ransom of Red Chief.
Ruthless People came out in 1986. The writer, Dale Launer, asked Elmore if he copied the movie Ruthless People in The Switch, apparently ignorant that the book had come out eight years earlier.
Whether Launer’s idea came from Ransom of Red Chief or, despite his protest, he had himself pilfered The Switch is not known. The kidnap idea of Ruthless People bears more resemblance to Elmore Leonard than O Henry.
In any event, Ruthless People and it’s plot twist killed a movie version of The Switch. Diane Keaton was lined up to play Mickey, the kidnapped Bloomfield Village tennis mom.
Here’s another example where proof, or at least a specific accusation is made. Elmore reports that Barry Sonnenfeld told him that Joel Silver, producer of Lethal Weapon II, admitted to Barry that he stole the opening from Freaky Deaky for the explosing toilet scene in Lethal Weapon II.