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
+```