I would say "Stop. Now start again and talk slowly." I think it is because they are wanting to say one thing (like 'I want to go outside') but their little brains are thinking to what they are going to do when they get outside, so they get stuck on a word.
Please please please don't do this. I have vivid memories of my dad doing it to me when I was little & it drove me nuts. In the end I just didn't bother telling him things because I would get that line every time I said something. I knew what I wanted to say, I just couldn't get it out & I'm not sure it was a speed thing (as in talking too quickly). I shudder at how many times my dad said "stop, think about what you are going to say & then say it", he even mentioned it in his speech a my wedding :eek:

Fruitwood, I am the same - still do it every now & then.

If you're really worried Tanya, get Emily off to a speech therapist to be checked as the others said. If you're not worried I think the best way to deal with it is to ignore it as bringing attention to it can make her more frustrated and possibly make it worse.