fix: allow registration with unverified email when verification is not mandatory

Previously, the registration process incorrectly blocked users from registering with an existing but unverified email, even when email verification was not mandatory. This caused a unique constraint error when saving the user.

This fix updates the `validate_email` method in `CustomRegisterSerializer` to:
- Allow registration if the email exists but is unverified, **only if verification is not mandatory**.
- Prevent duplicate registrations if the email is already verified.
- Improve error messages for better clarity.

This ensures smoother registration flows while maintaining email uniqueness where required.

Author: Mohmmde1

dj_rest_auth/registration/serializers.py

@@ -244,7 +244,8 @@ def validate_email(self, email):
                     raise serializers.ValidationError(
                         "A user is already registered with this email address."
                     )
-                else:
+                elif allauth_account_settings.EMAIL_VERIFICATION == \
+                    allauth_account_settings.EmailVerificationMethod.MANDATORY:
                     raise serializers.ValidationError(
                         "This email is already in use but has not been verified."
                     )