This week's book giveaways are in the Scala and Android forums. We're giving away four copies each of Machine Learning Systems: Designs that scale and Xamarin in Action: Creating native cross-platform mobile apps and have the authors on-line! See this thread and this one for details.
I'm not sure what exactly you mean to ask. If you want to know if the mock exams in our simulator can be taken only once, then the answer is No. You can take the exams more than once, a new performance report will be generated for each attempt you make.
You can use the Diagnostic test to assess your knowledge and recognize the weak areas. You can work on those topics and then go on to the practice exams. The final test can be taken once you feel confident to go for the actual exam. You will be surprised to find how close your score is to what you get in the actual exam.