problem

According to the task log of the mentioned tasks in the linked TS issues the reminder mail was sent and logged multiple times. The algorithm to determine if a user was notified is basically OK, only the check of the extra data and the extra data it self is not really robust / correct.

solution

The extra data must be with a textual key, and instead regex and searches for that ID the mysql JSON methods should be used to check for which user a notifications was send.