[FIX] mass_mailing : remove duplicate from blacklist form view

As blacklist email is unique and email is the only attribute,
it's a non sens to allow to duplicate a record.
+ To avoid import error on blacklist if import a duplicated address,
the create return the existing record instead of creating a new one
(which is not possible due to unique email constraint).
The duplicate was only editing the existing record.

addons/mail/views/mail_blacklist_views.xml

@@ -15,7 +15,7 @@
         <field name="name">mail.blacklist.view.form</field>
         <field name="model">mail.blacklist</field>
         <field name="arch" type="xml">
-            <form string="Add Email Blacklist">
+            <form string="Add Email Blacklist" duplicate="false">
                 <sheet>
                     <div class="oe_button_box" name="button_box">
                         <button name="toggle_active" type="object" class="oe_stat_button" icon="fa-archive">