Apparently it's very common, as my own daughter went through the same stage and I took her to a speech therapist. They do grow out of it, and my own little horror is testiment to that!
I was told by the therapist that what I was doing, telling my daughter to slow down when saying her sentances was wrong. The idea is to totally ignore the stammer altogether, as otherwise you're drawing attention to it and making it worse.
However, there's no harm in booking to see a speech therapist to put your mind at ease though.