From the Abridged Edition of the Dictionary of American Slang:
v phr by 1940s To challenge provocatively: The movie double-dared its audience to find sympathy in its dour or manic characters --Time/ …I double dog dare ya to find out which three! --Toronto Globe and Mail [fr a boys’ response to “I dare you!”, “I double dare you!”; double dog dare is still higher defiance].
That’s interesting, but I was talking about the phrase, not a game show with the same name. (And we all know how little TV game shows stick to original rules, i.e., they don’t even try; the familiar name is just the hook.)
While the TV show dates from the 70’s, the phrase is much earlier.
There was a big band song in 1938 *I Double Dare You */ by Terry Shand And Jimmy Eaton
And a movie Double Daring (1926).
For some reason, I thought that the double was more than a simple intensifier, as in the Dictionary of American Slang version.
Something on the order of “I dare you to do something, and if you won’t then I will.”
The example would be to double dare someone (say an initiate to a group or gang) to do something outrageous. Knock off a policeman’s had, steal an apple as you passed the fruit stand, down a drink in one swig. The idea would be that the newcomer would have to do it, and if complaining it was impossible to be shown up.