Registering with an existing username changes that user's password

[DJeJa003]

When attempting to register a new user with a username that already exists, the system does not return an error but instead changes the password for that existing user.

Steps to reproduce:

1. Open the application and navigate to the registration page
2. Enter an existing username (or register twice with the same name)
3. Enter any password
4. Click "Register"
5. Log in with the existing username and the new password

Expected behavior:
The system should reject the registration attempt and display an error indicating the username is already taken.

Actual behavior:
The password for the existing user is changed without any warning.

Environment details:

• OS: Windows 11
• Browser: Chrome