Francis Ford Coppola

James Caan agrees with Francis Ford Coppola -- there should have only been one "Godfather" movie. Of course ... when you hear his reason why, you'll see his opinion is a little tainted.

"Godfather" director Francis Ford Coppola says "Part II" -- which many consider the greatest sequel of all time -- was a MISTAKE ... insisting, "There shoulda only been one." Coppola was leaving Hemingway's Lounge in L.A. last night when we asked if he would ever pick up the Corleone storyline for…