Is this normal?

[Artsand crafts]

I had 2 of them like that. Both under 18 mos old. One of the banged his head to hard for my taste... The mother checked with his doctor and he said that was normal. Some kids are suppose to do it. Doctor said they weren't hurting themselves. I noticed they did it when trying to sooth themselves to sleep or during eating time. Even mom told me was normal I put a towel behind his head when eating.