The thing is, is that Doc's "double is" has been tracked by linguists...

[Mark Kohut]

Thomas Pynchon, holder of 7 OED first use entries, situates Doc's 'double is' 
before the 1971 that is indicated here. Not until late 80s for the Englishers hereon. Match
any experience of yours? 

Either another first use or a slight anachronism for effect. I vote for first use.

http://en.wikipedia.org/wiki/Double_copula