The current implementation of Expiration Policy notifications has the notification fire off any time the policy is matched. This means if Days Before Expiry is set to 30, then notification will happen 30 times (assuming cron shell is set to run nightly).

A better option would allow (eg) weekly notification, or perhaps just to notify once (or n times). This would require specifying the frequency in the Expiration Policy, and then some way of tracking how many notifications have been sent. (That could be done by adding source_co_expiration_policy_id to co_notifications.)