Once a candidate takes an exam, their registration is closed Candidates will only receive one Authorization to Test (ATT) per registration.
If a candidate would like to take another exam, they would have to reregister to receive a new ATT. This also applies if a candidate fails to appear for their exam for any reason or their ATT expires.
Candidates who will need to reregister may find more information in the article, What is the NCLEX registration process?