[FIX] mail: Don't create duplicate mail.notification

When more than one user are linked to the same res.partner and this user is assigned to a task or a sale.order
as many email notifications will be created as there are users linked to the same res.partner.
This won't send more than 1 email, but multiple mail.notification
records will be created.
This causes duplicate entries in the mail.notification table, and can
crash when an old unique constraint is still present in that table (from
earlier Odoo versions)

To prevent this, we deduplicate the notifications and order by notification
type to get 'email' first in case the user have different notification
type ('email', 'inbox').

How to reproduce:
   - Create a first user test1
   - Create a second user test2
   - Merge the contact test1 and test2
   - Create a customer customer
   - Set the user test1 as salesman
   - Create a new sale.order with customer as partner

Add a test to reproduce the pathological scenario

Solution
  add distinct on partner.id and order by partner.id,
users.notification_type

closes odoo/odoo#43862

Signed-off-by: Olivier Dony (odo) <odo@openerp.com>