My son did the same thing. some kids will grow out of it but I would have your child evaluated. The speech therapist calle my son's problem severe disfluency. He went to speech therapy for about 2 years and then we were able to use the speech therapist at our school. The therapist at school says the fact that we had him in early probably helped him to recover. His speech is fine now with the exception of having trouble not always closing his teeth when making "S" sounds. He's 10 now. I'm glad we went the extra step to be sure.
I see that several of the responses have said their child does the same thing and they will grow out of it. I believe that is true as well in most cases. However, repeating a word over and over is not the same as stuttering the beginning sounds. If this is the case, I would have your child checked out to be sure. Sooner Start can do it and then you'll know for sure.
In the meantime, I would do what someone else suggested and ask her to slow down and tell you again and just don't make a big deal out of it.