New user can have the same name with an existing user

[hunterhector]

Describe the bug A new user of this project will have the same name with the existing user.

To Reproduce Steps to reproduce the behavior:

1. Go to "All Users" page
2. Try add another user with the same name
3. You will see the new user appear there with the same name.

Expected behavior The server should reject this new user.

Screenshots If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• Work on all environment

Additional context Add any other context about the problem here.