website/docs: add example policy to enforce unique email address (#8955)

* website/docs: add example policy to enforce unique email address

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

* reword

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

---------

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

website/docs/policies/working_with_policies/unique_email.md

@@ -0,0 +1,19 @@
+---
+title: Ensure unique email addresses
+---
+
+Due to the database design of authentik, email addresses are by default not required to be unique. This behavior can however be changed by policies.
+
+The snippet below can as the expression in policies both with enrollment flows, where the policy should be bound to any stage before the [User write](../../flow/stages/user_write.md) stage, or it can be used with the [Prompt stage](../../flow/stages/prompt/index.md).
+
+```python
+from authentik.core.models import User
+
+# Ensure this matches the *Field Key* value of the prompt
+field_name = "email"
+email = request.context["prompt_data"][field_name]
+if User.objects.filter(email=email).exists():
+  ak_message("Email address in use")
+  return False
+return True
+```

website/docs/policies/working_with_policies/whitelist_email.md

@@ -2,8 +2,7 @@
 title: Whitelist email domains
 ---
 
-To add specific email addresses to an allow list for signing in through SSO or directly with default policy customization,
-follow these steps:
+To add specific email addresses to an allow list for signing in through SSO or directly with default policy customization, follow these steps:
 
 1. In the Admin interface, navigate to **Customization > Policies** and modify the default policy named `default-source-enrollment-if-sso`.
 
@@ -11,13 +10,12 @@ follow these steps:
 
 ```python
 allowed_domains = ["example.net", "example.com"]
-current_domain =request.context["prompt_data"]["email"].split("@")[1]
-if current_domain in allowed_domains:
-  email = request.context["prompt_data"]["email"]
-  request.context["prompt_data"]["username"] = email
-  return ak_is_sso_flow
-else:
-  return ak_message("Access denied for this email domain")
+
+current_domain = request.context["prompt_data"]["email"].split("@")[1]
+if current_domain not in allowed_domains:
+  ak_message("Access denied for this email domain")
+  return False
+return ak_is_sso_flow
 ```
 
 This configuration specifies the `allowed_domains` list of domains for logging in through SSO, such as Google OAuth2. If your email is not in the available domains, you will receive a 'Permission Denied' message on the login screen.