Common Admission Test(CAT) is being held just once in a year so you will have chance to take up the CAT exam even if you have already 

Took it there will be no problem if u mentioned that you have taken CAT  before or not as you are already eligible to take the test

If you have made a mistake in filling the registration form then it may be a problem for you in the process of selection as the selection criteria strictly based on marks and the details but in this case there will be not that much problem so you can happily attend to your CAT exam