[bug] User receives multiple identical notifications

[kkreitmair]

If a user is in several organizations,
they will receive multiple identical notifications.

The reason for this is that the database query, which determines which user should receive the notification, can return a user multiple times.

Steps to replicate the issue:

1. Do a fresh installation of openwisp-controller as described in the docs and launch the development server.
2. Re-enable the "default" notification type by commenting out the lines 268 to 272 in openwisp_controller/config/apps.py.
3. Log In to the admin site with the user created during installation.
4. Create a second organization.
5. Make the user organization user of the two organizations (is_admin is not required).
6. Create a nottification type setting for the type "Default Type" with organization default, web notification checked and email notification checked.
7. Then create a notification with the following command: env/bin/python tests/manage.py create_notification
8. This user will then receive the notification twice.