The way that the mock exams are structured is in a 30 question format, and these are randomly selected for each test. So it is possible although unlikely that you might see the same question twice. Results of previous tests are listed and can be reviewed. Unfortunately we currently don't have a method of eliminating previously attempted questions, although this is something we will look at for future updates.
Each test is different as it pulls a random 30 questions from the 570 question bank. Therefore no two tests are the same, and there is only a very small chance that a question may be repeated.