A student can complete the same unit multiple times. This is common with Units requiring refresher training (or re-assessment). First Aid and CPR Units are good examples.

While we're on this topic, PowerPro has a built-in report ("Reports>Client Reports>Refresher List") listing all students who are due for a refresher:

 

For the Refresher List report to work, please ensure the relevant Units and Modules have the Requalification period set as per example below: