I was wrong. The quote appears not twice, but thrice. I managed to get hold of the transcript of the movie. I have extracted the lines for the sake of ease.
http://www.jgeoff.com/godfather/gf1/transcript/gf1transcript.html
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MICHAEL
My father made him an offer he couldn't refuse.
KAY
What was that?
MICHAEL
Luca Brasi held a gun to his head, and my father assured him that either his brains -- or his signature -- would be on the contract.
~~~~~~~~~~~~~~~~~~~~~~~~~
JOHNNY
It's too late, they start shooting in a week
VITO CORLEONE
I'm gonna make him an offer he can't refuse.
(then)
Now you just go outside and enjoy yourself, and ah, forget about all this nonsense. I want you to leave it all to me.
~~~~~~~~~~~~~~~~~~~~~~~~
FREDO
Hey, Mike, are you sure about that? Moe loves the business -- he never said nothin' to me about selling...
MICHAEL
Yeah -- well I'll make him an offer he can't refuse.
(then, after Tom hands Michael a set of contracts)
See, Johnny -- see we figure that entertainment will be a big factor in drawing gamblers to the casino. And, we hope you'll sign a contract to appear five times a year.
The God Father is one of my all time favorite movies.