New user can have the same name with an existing user

**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.