The Random category and the Simulated Exams pull questions from all categories. So if you work in the regular categories and then the Simulated exams or the Random category, chances are that you will see some repeat questions. If you are working exclusively in the regular categories, you should not be seeing repeat questions unless you exhaust all the questions in each category, then the queue will start over and you will start to see some of the same questions again. You will however, see questions you have seen before in the Missed Question tests category.