I wouldn't call them normal or abnormal. I would call them common, but something that ideally, if it occurs, the owners needs to try to work out the triggers to prevent it happening again, because I imagine that its uncomfortable and maybe even distressing for the rabbit involved.
Not all flase pregnancies can be stopped, but some can (if like triggered by certain smells, or other rabbits, or if its a healthy rabbit and just not spayed, etc).