Duplicate email can still add new users

[hongtring]

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• Nextcloud Server is running on 64bit capable CPU, PHP and OS.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

Duplicate email can still add new users

Steps to reproduce

1. Sign in
2. Profile Admin > Users
3. Add new user
4. Duplicate email already on the system
5. Save

Expected behavior

One Email can only register 1 account

Installation method

Community Manual installation with Archive

Nextcloud Server version

25

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.1

Web server

Apache (supported)

Database engine version

MariaDB

Is this bug present after an update or on a fresh install?

Fresh Nextcloud Server install

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

.

List of activated Apps

.

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

[solracsf]

Nextcloud allows one unique username, not one unique email. To accomplish that, set the username == email.

[pierreozoux]

I just spent several hours to debug a cal issue due to a duplicate email. I think Nextcloud should enforce unique email, or at least this could be an option.

[joshtrichards]

Duplicate of #17146