Originally Posted by Altenweg
It's common. She's acting out, she's not getting what she wants and hitting, to her, is the only option.
Have you tried giving her a time out, or taking away a favorite toy. Just because it's common, doesn't mean it's acceptable, it's time to put a stop to this behavior before it gets out of hand.
Now remember the rule of thumb for time out's, 1 minute for every year of the child age. Two minute time out would not be inappropriate for your child. Put her in a spot like a chair in the kitchen, tell her she's in time out, hit the timer on your microwave for 2 minutes. When the time is done, ask her if she knows why she had a time out, if she doesn't, then tell her. Time out is usually done after at least one warning to stop the behavior.
Good luck.