M.B.
I had to giggle when I read this because now my daughter is 20 months and it seems like such a long time ago that she did this.
Yes it's perfectly normal. She's becoming more mobile in the daytime and she's practicing those new moves when she sleeps.
If she's staying asleep the first time she bangs her head, then it seems as though you're moving her back is what is waking her up the second time. She isn't going to hurt herself if you leave her be and that is exactly what I would do and did. Just like all the other phases, someday this will just be a distant memory for you. Don't worry.