[FIX] mass_mailing: deduplicate recipients within campaign by default

It is safer to avoid duplicates within a campaign,
so this should be turned on by default. Especially true
because campaign are often created without opening the
form view (quick create)

Also improved the description of the boolean option,
and made it editable in the form view, in case someone
wants ho really create a multi-step campaign.

addons/mass_mailing/models/mass_mailing.py

@@ -153,10 +153,10 @@ class MassMailingCampaign(models.Model):
     mass_mailing_ids = fields.One2many(
         'mail.mass_mailing', 'mass_mailing_campaign_id',
         string='Mass Mailings')
-    unique_ab_testing = fields.Boolean(string='AB Testing',
-        help='If checked, recipients will be mailed only once, allowing to send '
-             'various mailings in a single campaign to test the effectiveness '
-             'of the mailings.')
+    unique_ab_testing = fields.Boolean(string='Allow A/B Testing', default=True,
+        help='If checked, recipients will be mailed only once for the whole campaign. '
+             'This lets you send different mailings to randomly selected recipients and test '
+             'the effectiveness of the mailings, without causing duplicate messages.')
     color = fields.Integer(string='Color Index')
     clicks_ratio = fields.Integer(compute="_compute_clicks_ratio", string="Number of clicks")
     # stat fields

addons/mass_mailing/views/mass_mailing_views.xml

@@ -691,6 +691,7 @@
                             <field name="name" string="Campaign Name"/>
                             <field name="user_id"/>
                             <field name="tag_ids" widget="many2many_tags" options="{'no_create_edit': True}"/>
+                            <field name="unique_ab_testing" groups="base.group_no_one"/>
                         </group>
                         <notebook>
                             <page string="Tracking">