My DD says exactly the same all the time 'What did you say' a million times a day. I am amazed that someone else has the exact same thing. I even considerded getting her hearing tested!!

I have found listening to myself, that is what I say to her when I don't understand or didn't hear what she said. The other thing is that she used to say "I think" at the end of all her sentences. Something else she picked up from me. That has stopped and been replaced with "what did you say". So I think it is a stage. They just pick up these little quirks in language get fixated with it and then move on to the next thing. So that I don't listen to myself endlessly repeating what I said all day I try to not anwser it, but sometimes its just habit for me that I anwser her.