GMAT Prep has big question bank. However, there is a small overlap between CAT 1 and CAT 2. If you appear for any of those tests more than once, there is high chance of getting repeat of a few questions (may be 3-6) for each section -- still, it depends on how do you perform. If start of your one test is low, you will get more of low scoring questions in that test. If the start of the other test is high, you will get more of high scoring questions in that test. In anyway, there is every chance of getting a few repeat questions.
Anyway, there is no harm in taking the CAT more than once ... it will help you in setting pace and getting acquainted with real exam scenario. Though, I must inform here, questions in the prep s/w are from the past exams and those are mainly of mid-scoring range. You may expect tougher questions in the actual GMAT.