Two things:

1) You can take each of the two free GMATPrep tests more than once. You will likely see some overlap in questions, but since this is an adaptive test, you will see fewer questions that repeat if you improve your abilities substantially. In other words, if you are getting more difficult questions right the second time you take the same GMATPrep test you will see fewer repeat questions. Since you have only studied for 15 days, I would say that you should probably take the official GMATPrep test 1. If you score really well then you are on your way and you can always purchase the two additional GMATPrep tests later. If you do not score as well this time then after you have studied some more when you return to this test for a second try you will likely see few repeats.

2) The Veritas Prep Free Practice Test should be on your list. The evidence that I have seen indicates that it is pretty accurate in terms of matching up with the score that people are getting o the GMATPrep and on the official GMAT. You can take this one now to see where you stand, or if you take the official GMATPrep test 1 now then be sure to keep this one in you rotation for later! https://www.veritasprep.com/gmat/free-gm ... tice-test/