It really depends on your performance. Ideally, if you've run through all 6 CAT tests, your performance should be much better the second time around, which means you shouldn't see that many repeats. Also, if you haven't reviewed your CATs extensively, you may not recognize the repeats that do appear.
If you should happen across a problem you've seen before, you can work to reduce score inflation by taking the full time to answer the question. Also, if you missed the problem before, choose a wrong answer this time, too, unless you know for a fact that you would get it right this time around.
I guess the short answer is that seeing things for the second time is likely to reduce the accuracy of your score, but by this point you should have a decent sense of your ability. If you have already completed 8 or more CAT tests, you can use new exams to work on timing and problem identification, and to review areas that are still giving you trouble.