You can always retake for the sake of practice and pacing, also learning new problem when you experience them. But ignore the final score since it might not be a good reflection.

When you experience a problem you have seen, e.g. a quant problem... wait at least 2min, and then continue so your pacing practice at least is intact.

hope this helps!

Having taken so many practice tests you should have an idea of the range that you might score in on test day. I am sure that you have studied since your last test and you want to check for improvement.

Unfortunately, only the GMATPrep official tests are accurate predictors, while the other unofficial tests are good to learn from and to practice your skills.

Having taken each test 3 times you will start to see many repeated questions. It may be worth it to keep going just to get to the new questions that you have not seen. But pay no attention to the score you receive. If you get the new questions right that might be a good sign!

If you are looking for a Quant predictor you can take the GMAT Focus - also from GMAC. It has official questions and it adapts to you so that is a plus. It is 24 questions and will give a small range of Quant scores that you can expect to earn on test day. It does cost something though $25 for one test. However, the interface is more useful than GMATPrep's interface.