Hi Ashamax,
When you complete the tests, the results are valid for one year, if you have applied and completed the tests in the past year the test results will still be valid and you will not need to redo them.
If you have never applied before or last applied more than one year ago then you should get in touch with kenexa at the email address they gave you in the acknowledgement email that will have sent to you immediately after you applied.
Hope that helps a bit.