We should not be able to create users with the same username

[marwa-benhassine]

Describe the bug

It is possible to create users with identical usernames if the first letter is in different case (upper or lower case) allowing duplication of names.

To Reproduce
Steps to reproduce the behavior:

1. Go to the user page
2. Click on the new user button
3. Enter a username with the first letter in either (uppercase or lowercase)
4. Click on the Create button
5. Try to create another user with the same username but with a different case for the first letter (uppercase or lowercase)
6. Notice that the system allows the creation of the duplicate username
7. Repeat the same steps during the Signup Process.

Expected behavior

An error message should appear when trying to create a username with the same username: Username already exists.

Screenshots

If applicable, add screenshots to help explain your problem.

Username.already.exists.mp4

Desktop (please complete the following information):

OS: Windows

Browser: Chrome/Edge/Firefox
.