With the recipient_policy and auto generate mail attribute disabled, there's no check if a mail attribute is already in use.

It's possible to check for duplicate addresses in the alias field when the validation method is set to extend. But this check is missing in the validate_mail() method.

In the end this can lead to duplicate mail addresses and unpredictable things with mailboxes if you try to rename the duplicate mail attribute (because the mailbox gets renamed as well).