New Users

For most of the features we need to test it is important to find users who's job and role match who the feature is designed for.

Repeat Users

Can you use the same test user multiple times? It depends. The question is what are you trying to learn.

If you are testing if a new user understandings intuitively how the navigation of the application works, do not return to the same test user with the improved navigation mockups. It is better in this case to find a new and fresh test user who has never seen that navigation before. 

On the other hand, if one the first usability test you studied how the user interacts with the navigation feature, and on the second usability test you studied how the user interacts with the vitals trends these are two separate parts of the application. You could return to the same user and perform the second test.