[FIX] im_livechat: no duplicated messages

Before this commit, when using website livechat with multiple tabs,
it may duplicate messages in the chat window.

This happens due to registering messages twice:

- once, from `/mail/chat_history`
- second, from receiving notification from master tab, via local
storage.

This commit fixes the issue by not registering messages that have
already been registered (i.e. unique ID).

opw-2052960

closes odoo/odoo#36370

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/im_livechat/static/src/js/im_livechat.js

@@ -232,7 +232,12 @@ var LivechatButton = Widget.extend({
             is_note: data.is_note,
             customer_email_data: []
         };
-
+        var hasAlreadyMessage = _.some(this.messages, function (message) {
+            return message.id === msg.id;
+        });
+        if (hasAlreadyMessage) {
+            return;
+        }
         // Compute displayed author name or email
         msg.displayed_author = msg.author_id && msg.author_id[1] ||
                                this.options.default_username;