Don't buy them again! There is only the one database of questions that leads to sets of 6 tests.
You can start a new set of 6 tests (without paying anything more - I think you have access for 1 year from the time you bought them). You may see questions that you saw on the first set of 6 - but the test results can still be valid as long as you follow a few guidelines to minimize the chance of artificially inflating your score via question repeats.
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 *completely honestly* about whether you think that you would get the question right if it were a totally new question. If so, get it right. If not, get it wrong on purpose. This way, you won't artificially inflate your score.
There are also two free GMATPrep tests (if you haven't already done them) and two more that you can buy (
www.mba.com). You don't necessarily need that many practice tests - just in case you want to have some new ones.