My two cents and some people might disagree with me.
Try not to repeat the GMATPrep CATs. They are the best indicator of your actual GMAT performance and should be used extremely carefully. I know that if you repeat, you get new questions. BUT - the questions that end up coming again might be the critical one's that push the score high (artificially).
Some reasons for getting a low GMAT score, even though the GMATPrep score was extremely good:
1) Repeating the GMATPrep. (I would say, if absolutely necessary, you can at best repeat ONCE. No more than that)
2) A lot of questions from GMATPrep and other CATs are discussed to death on the forum. If you end up getting 5 questions that you have discussed for a week on the forum (you remember the answer), and if these questions are those critical questions that move you a notch up on the score chart, final score on the CAT is not a true reflection of the skills.
Good Luck everyone.
All things are possible to those who believe.