Why do you think O'Connor chooses to bring up The Misfit a second time in "A Good Man Is Hard to Find"?
What begins as an ordinary and comic story of an American family heading out for a typical 1950s vacation in the car with Mom, Dad, two children and a difficult grandmother hiding the family cat will become a grotesque tragedy and story about the power of grace. O'Connor foreshadows the tragic turn of events in the story by bringing up the Misfit twice before he and the family have their encounter. By having the grandmother function as the one to speak of him most, O'Connor establishes a relationship between the two of them even before they meet. The grandmother mentions him in the first paragraph of the story, warning Bailey about him, and later discusses him with Big Sam at the diner. The double mention of the Misfit helps the reader to anticipate that he might somehow get involved in the plot of the story and makes it far less strange when he pops up in the lonely woods where the family has flipped the car into a gulch on the side of the road.
The grandmother's second mention of the Misfit, in the context of a good man being hard to find, shows the banality and superficiality of her definition of a "good man." She calls Red Sam a good man when he tells her he allowed "some boys from the mill" to have their gas on credit because they looked reputable. Later, she will call the Misfit a good man three times, saying she can see he doesn't come from "common blood." She mentions two more times that she can see he is a good man. But at this point, she isn't seeing him. It isn't until the very end, when she says "you're one of my own children" and reaches out a hand to touch him, that she really sees him, and at this point, she is beyond banal characterizations of who comes from good blood.
The Misfit is brought up a second time by the diner owner. This serves two purposes. The first is to establish his continued presence within the story. He will become an important character at the story's climax, and O'Connor allows him to almost haunt the narrative.
The second is to provide the grandmother and the diner owner an opportunity to expound upon the title of the story: a good man is hard to find. In their own simple way, they explore the story's central themes of good and evil. The Misfit represents evil in the story, and the grandmother represents a tainted sort of good.
The Misfit is brought up a second time both to further promote his existence in the story and to allow the characters to discuss the nature of good and evil.
The Misfit is a key figure in "A Good Man is Hard to Find." He almost seems to haunt the narrative as he is featured in both the news bulletin and the conversation with the diner owner. The key phrase spoken is the title of the story, "a good man is hard to find." The grandmother will use this idea to plead with The Misfit for her life at the end of the story. That is why he keeps coming up even before he encounters the family.
He serves as the inverse of the grandmother. He is evil, but he is completely at peace with himself. The grandmother is not a bad person, but she is delusional. A good man is hard to find when your life is in the hands of an evil man. The Misfit's importance to the story is why he is brought up so often.