Just because a word or phrase can be assigned a "first recorded" appearance in print doesn't mean it wasn't already in use orally.

Even Guest Q, while agreeing this the premise that Al Capp gets credit for first-use of "double whammy," allows that the word "whammy" had already been in use within the context of baseball, meaning (as I understand it) a "jinx" or "curse."

Who's to say Mr. Capp wasn't using a phrase he'd picked up somewhere, perhaps even a phrase already in fairly wide use among one or more of American society's less visible subcultures (black folks, po-white hillbillies, reeferheads, jazz musicians, etc., etc.) Until the fairly recent spread of electronic communications, the unique jargon or slang of each of these groups would, almost by definition, *never* be published in books or mainstream periodicals.

The "single" whammy, if not the double one, is apparently known to have a longstanding unpublished history, so who knows?