No one knows the basis that Google has for deciding duplicate content. However, it's probably something to do with how much similarity exists in two pages, even if they have their own (slightly different in your case) URLs.
Given this, what strikes me as bring wrong/risky is your use of the same Idea cartoon on the three pages. It could be wiser perhaps to make each one a blown up version of one that's drawn from your obviously extensive collection of cartoons on the specific topic.
Duncan
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