Definitely don't buy our CATs a second time. There's only one big database of questions - there aren't multiple sets.
Before we talk about what to do, I want to make sure of one thing. You don't really get better when taking a CAT. Mostly all you're doing is gauging your progress to that point, and then using the data to help set up your further study plan.
Think of a pro athlete: when Serena Williams is actively playing a match, she's not learning how to become a better tennis player. She's just trying to win. She gets better
between matches, when she analyzes how things went during her last match and devises drills and practice sessions to fix the specific areas she's identified as problematic.
The same is true for GMAT study. So, if you are taking a ton of practice tests in hopes of having that improve your score, stop! Most of your learning / improvement comes
between tests.
Next, you can still take both GMATPrep and MGMAT CATs with repeats as long as you follow a few guidelines to minimize the chance of artificially inflating your score via question repeats. Unless you have taken the previous CATs within the last few weeks and practically memorized the problems, the chances are good that you won't remember the questions you've seen before (and there are lots of questions in the database, so you'll see plenty of new ones, too).
First, anytime you see a problem that you remember (and this means: I know the answer or I'm pretty sure I remember the answer, not just "hmm, this looks vaguely familiar..."), immediately look at the timer and make yourself sit there for the full length of time for that question type. This way, you don't artificially give yourself more time than you should have.
Second, think about whether you got this problem right the last time. If you did, get it right again this time. If you didn't, get it wrong again. If you *completely honestly* think that you would get it right this time around if it were a new question (even though you got it wrong last time) because you've studied that area and improved, then get it right this time. But only if you are really confident that you've learned what you need to learn to get it right on your own.
Have you also done the GMATPrep tests? There are two free ones (at
www.mba.com) and you can buy two more for $40. They don't have the same kind of analysis / data that our tests have, but they are the best gauge of your current scoring ability (since they're made by the people who make the real test).
But remember that the goal is not simply to do a ton of tests! You're not actually getting (much) better by doing a bunch of tests without thoroughly reviewing and devising a study plan based on what you see in the results. :)