<p>According to one book I have read, one thing they check for is whether the same fingerprints show up on both test books. In one case in which a guy never got his scores reported from his second test, it was because his fingers had somehow never touched the test booklet. </p>
<p>But all of the above is just to relate an interesting story I read about once. In actual practice, your score can increase a LOT (I have seen one case, on successive test dates) and College Board seems not to care at all. Maybe the test-taker had the flu the first time he took the test, or whatever.</p>