As tempting as it might be to say, "Dad, you just said that," that's not helpful. If his statement seems to beg for a response, just say, "Oh, OK, Dad," or "Thanks for telling me," in a positive way, and move onto something else. When you remind him he's repeating himself, what you're really doing is lowering his self-esteem. It's best not to make a person with Alzheimer's disease feel more impaired, more ashamed. It's better to respond in ways that will, as much as possible, help your father retain his self-esteem.
Of course, that takes patience. If your dad keeps asking a question over and over, what he's asking is important to him -- even if it's not equally important to you -- and you have to respect that.
Sometimes it's a repetitive question, like "When is lunch?" You can make a card and place it near him: "Lunch is at 12:30." Write down something that acts as a cue. Some assisted living communities place a sign by the mail area that says, "The mail will arrive at 3 p.m."
If he's making repetitive phone calls, consider putting a note in large type by the telephone: "Do not call. Joyce will call you at 1 p.m."
It helps to remember what's really going on. You can compare the disease to a tape recorder that's turned off; nothing gets recorded. No matter how many times you present new information, he may not remember it because it never got recorded. On the other hand, the "tape recorder" in your dad's mind was turned on when he was younger. That's why he probably recalls old songs and all those other things from when he was younger.
Try not to get frustrated with your father. Instead, get frustrated with the disease. It's not something your father has chosen. Think of it as a gift that he can still speak and ask questions.