the general user settings uses the user_id to store the settings when the notification mails will be send out.
This features was committed here https://github.com/ezsystems/ezpublish-legacy/commit/81a4209bbe195a567e7bc198f25966c4de17745f
But if an object will be generated an email item into table
eznotificationcollection_item it will be only used the email (address) to store the content.
But this makes no sense. Ez stores the settings with the user_id but not the output.
If we have an ez installation where the email address is not unique (we have one) the 2 users with the same email and different general diggest settings are only get 1 email instead of 2 (for each user one)
It would be nice if ez will store an additional field user_id n eznotificationcollection_item, too.