It sure seemes like some of these cartoonists get together for lunch and brainstorm their content.
Lots of repetition or similar styles for a topic (Social Security, Gonzales "torture", Tsunami)
I doubt that they get together, but can't rule out a bit of plagiarism. Sometimes they're just hitting the most topical themes. I see the repetion when I'm collecting for my next post. I decided not to filter out toons that are almost identical to others...not wanting to judge which one's the "best".