userName is not unique

### Description of issue
sign in with 2 same user name
login accept the 1st user name

### Expected result
during sign in username should be unique and validation should be done

### Actual result
signned in with 2 same username

### Steps to reproduce
1.
2.
3.

```
Path: /pages/signup.html
Browser: Chrome 112.0.0.0 on Windows 10
Viewport: 1503 x 900 @1.5x
Language: en-US
Cookies: Enabled
```

[Open in BrowserStack](https://www.browserstack.com/user/try-live?url=https%3A%2F%2Fdeploy-preview-1--thamizhan-unavagam.netlify.app%2Fpages%2Fsignup.html&os=Windows&os_version=10&browser=Chrome&browser_version=112.0&resolution=1503x900&speed=1&start=true&ref=netlify-source)

Submitted from [Netlify Deploy Preview #1](https://deploy-preview-1--thamizhan-unavagam.netlify.app/pages/signup.html)