Duplicate notifications sent if multiple messages triggered in the same page load

Problem/Motivation

If you have a single page load (or in my case, hook_cron() invocation) that's triggering multiple group_notify events, users can get duplicate notifications for the same group content.

This is because of how the protected NotifyGroupNode::$recipients array is handled.

1. group_notify_notify() gets the content plugin: $plugin = $entity->getContentPlugin();
2. group_notify_notify() calls $plugin->addRecipient($member); once for each group member.
3. group_notify_notify() calls $plugin->sendNotification($params);, which inspects $this->recipients for the list of users to notify.

If this only happens once in a given request, all's well. But when a 2nd node is updated in the same request, we get into trouble. Step #1 above returns a link to the same plugin object. It's already got all the recipients from the initial notification. But step #2 happily adds them again, and step #3 means we get multiple emails for the 2nd node.

If there was a 3rd node being updated in the same request, everyone would get 3 emails.

If the nodes are in different groups, it's even worse, since it means that notification emails leak content to users who aren't members of the group. E.g. node 1 from group A goes out to everyone in group A. Then node 2 is updated in group B, all users of groups A and B see the emails for node 2. Folks in both groups see it as duplicates. But folks only in group A see it as access bypass.

Steps to reproduce

1. Install group + group_notify (8.x-1.5 or 8.x-1.x-dev).
2. Create a group type configured to use group_notify for a specific node type.
3. Create two groups.
4. Subscribe a user to both groups.
5. Either programmatically create a node for each group in a single page load, or in my case, update both nodes in the same page load to set them to be published and use #3164688: Forced notification when a node is created or updated via API (not via UI forms) to get group_notify to send notifications.

The user should get 2 emails (one from each group). In reality, they'll get 3.

Proposed resolution

Clear out the recipients before computing who gets notified for a given message.

Remaining tasks

1. Do it.
2. Tests?
3. Reviews / refinements.
4. Commit.

User interface changes

Nope.

API changes

Maybe we should add NotifyGroupNode::clearRecipients() so group_notify_notify() has a way to clear recipients for each distinct message?

Or we could always clear $this->recipients inside NotifyGroupNode::sendNotifications(). Probably that's better (and no API change).

Data model changes