Yes it's completely normal...my little girl does it sometimes too...they don't know the difference between right and wrong yet, therefore this is something they need to understand and be taught. Everytime he/she hits take thier hand and just say "no, not nice, be gentle" and make sure your consistant with him/her... well you don't have to say that exact thing but that's what I do and then my daughter will pet me instead and understand that it's not right...anyways good luck!!
It is usually learned form someone else thats what babies do ,copy. check out who is having a whack at him or teasing him,