Url aliases and redirect sources should not cross each other

I belive that redirect sources should not duplicate existing entity aliases.

Because there is no sense to have a content which is never accessible and redirects to another content.

I haven't find any validations during creating/updating a content to check this situation. Regarding results on following request it's possible:

SELECT *
FROM url_alias a
JOIN redirect r ON r.source = a.alias

I understand that 'redirect' module executes first, and redirects if it's needed. However believe that keeping duplicates in aliases may cost some problems.