When prepping for the MCAT, it can be tempting to go back to already completed quizzes or exams to "assess your progress". However, we heavily recommend against this for 2 reasons.
The first is about how we learn and recall information. If you have already seen a problem, completed it, and reviewed it, it is quite possible that, when answering a question a second time, you will recall some aspect of the last time you saw the problem. This can work for or against you: you may remember some aspect of the explanation and be able to pick the right answer for this specific question even without deeper understanding, or you may even remember the error you made last time and make it again because it is familiar. This may not be something you realize is happening, but, it is one of the main reasons why re-quizzing on identical content is not a meaningful measure of improvement. Even knowing this, it may be tempting to go back-just to make sure you really get "how to answer that question the right way this time!"
This is where the second reason comes into play: you will never see the items you have seen in prep on your actual MCAT exam. The MCAT does not repeat items, and it actually makes a point of constantly using new scenarios and setups. It's a tough critical thinking test, and it makes sure to maintain that reputation! In prep, it can be tempting to over-focus on specific questions you've struggled with, and want to work those problems over and over. Just remember: you will never see those problems on the exam; but, you may see other problems that rely on the same underlying content and critical thinking skills.
For this reason, working new problems and exploring a wider breadth of how the same content and critical thinking skills can be tested, rather than fixating on a presentation you've seen before, is a more effective way to prepare for the real MCAT.