Why “Many and many A year ago”, not “Many, many years ago? Are they same?
Solution 1:
What RyeBread said.
"It was many and many a year ago, in a kingdom by the sea" is the opening of a great poem by Edgar Allen Poe. Anabel Lee
Presumably Dick Cavett assumes that anyone over a certain age educated in the US (i.e. those who were viewers of his TV show) read the poem in school, and will understand the reference.
Solution 2:
He stole the phrase from Poe. Just using a famous quote as an attention getter. I personally think people should have their own style but some people like it.