No, your scores will not be affected by having taken the test before. However, both scores will be sent to any grad school you apply to, not just your best, and most grad schools consider some sort of average of the two I'm told. Bottom line, if you took it once and think you can do better, it is your money and as long as you do do better (I would say significantly better...if you score the near same that will just verify your lower score in the minds of the admissions committee) it will not hurt. That said, do not test the test "for practice." Download practice exams from the Harvard site and others and use those for practice...you don't want your "practice" test to be the reason you don't get into grad school.
By the way, the same thing goes for the general GRE. Go into it looking to take it once and only once if you can.