Notification Dispatcher job sends duplicate content approval e-mail notification

Description

Steps to reproduce

1. User A is a reviewer of content A at step 1.
2. Set Content A ready to review.
3. Open the mail box of content A > Get an email notification (OK so far).
4. In admin mode, manually start the Notification Dispatcher or wait until next job runs.

Expected
After the Notification Dispatcher job runs, no more email notifications should be sent to the user because the user already got a notification.

Actual
Each time the Notification Dispatcher job runs, the user receives an email notification from the same content approver.