This article describes a scenario when a user is trying to release a quarantined email from an active summary notification but the portal displays an error message :
The email cannot be released. Item already Released
Email Security.cloud
For security reasons, you can only release an email once from an active summary notification, in order to prevents a malicious user from releasing an email multiple times to launch a denial of service (DOS) attack.
If you are required to release the email once again, It can be released it from the Quarantine portal or the quarantine administrator can release it for the user from the quarantine portal.