Hi HelloKitty,
The Kaplan free practice-test is a one time deal. Re-taking it will result in you seeing the same questions, so you won't accomplish much.
That said, you should still take the test! Our free online diagnostic is just that: a diagnostic. It's meant to help you figure out what you need to work on and to isolate your strengths and weaknesses. I wouldn't worry too much about 'wasting' this test, since even a poor score will be the first step towards earning a great one! In addition, there are plenty of other tests out there--and even if you run out of free ones, the 6 tests included with Kaplan's GMAT premier will only run you $30, or $5 per test.
Good luck on your tests, and feel free to PM me with the results if you need any help figuring out how best to use them to shape your studies. And thanks for choosing Kaplan!