The installable GMAT Prep file you download from mba.com will be the
same every time. However, there are 2005 and 2006 versions of GMAT Prep. You might come across some repeat questions while doing 2005 and 2006 GMAT Prep Tests, but won't see too many of them. See the link in the bottom of this post for more details.
You can re-do the test, but you might see repeats. This software chooses its questions from the pool of questions. GMAT Test 1 and GMAT Test 2 inside GMAT Prep shouldn't contain duplicates in them.
Some people retake GMAT Prep tests a couple of times to make sure they solve most of the questions from the software question bank.
Have you tried PowerPrep? It is an older version of GMAT Prep. See this thread for more details about all versions of GMAT Prep software:
gmatprep-gmat-prep-powerprep-power-prep-free-tests-77548.htmlbigboymaximus
Does anyone know if the sample tests from GMATPrep are all the same? Like If I was to sign up and download them again to get a different test? Or if I re-did the test would the cat change around the questions.?
Thanks