I can say from first hand experience that it does happen, and is not as rare as some may think. I've seen people come into an ER, and because they don't know if they had an CT scan (try a confused elderly person), or even that they may have had the test but you can't get at the results, then the test is repeated.
It doesn’t happen that much. If it does you are probably going to have to pay for it. However, In an emergency situation all bets are off. That is not what O was talking about. In a regular clinical setting you are not going to get another MRI for the same symptoms if you recently had one.
Under Obamma’s plan you probably will not get another one for diagnosis of a new illness, because you had one for a previous illness.