I think cartoonists can come up with the same idea without ever knowing each other. If people are getting some of the same news, reading the same books and living in the same culture, I think it happens.
One time in the 90’s I was visiting New York with my family and my Dad let us stay a couple of days extra so I could speak to a famous designer/illustrator, Seymour Chwast and also drop off my cartoon portfolio to the New Yorker. Nothing ever came of that trip. Seymour told me to go to Illustration school and New York and the New Yorker just gave me an official New Yorker poster note with the word, “Sorry.” written on it. I don’t know if that was a message as in apology or as an adjective for the work that was in my portfolio.
One of the comics that I presented that day was a simple cartoon of a pirate walking to his pirate ship from a parking lot. He has a peg leg and just parked in the handicapped space. It is a simple idea. FLASH FORWARD to 2002 or so. I get a book about creativity from New Yorker editor, Robert Mankoff, called “The Naked Cartoonist.” I start reading and there on page 22 is a Pirate with a peg leg parking his rowboat on a handicapped spot. Probably just a coincidence but it stopped me in my tracks.
Caption: Moses parts his hair.
I know what you are thinking. Hey! That's a Far Side cartoon! You are taking Gary Larson's ideas and making them your own!
Actually, Gary's cartoon was published after mine did so I guess great minds think alike.