There are two versions of the gmat prep, the old one and the new one.
The questions in both are almost different so you can give the older ones i.e gmatprep 1 and 2, twice and new ones twice. the tests by other prep companies with the exception of mgmat are not on the lines of actual GMAT and hence best be avoided. IMHO, you must lay more stress on issues such as, detailed review after each test, time management, test taking strategies rather than giving far too many tests
Cheers and good luck.
P.S : If you want to score very high in quant, must do gmatclub quant tests. They are 24 carrat gold.
Fire the final bullet only when you are constantly hitting the Bull's eye, till then KEEP PRACTICING.
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Three good RC strategies three-different-strategies-for-attacking-rc-127287.html