Another attempt at single ticket per email.

bemade_helpdesk_one_ticket_per_email/__manifest__.py

@@ -24,7 +24,7 @@
     'author': 'Bemade Inc.',
     'website': 'http://www.bemade.org',
     'license': 'OPL-1',
-    'depends': ['helpdesk'],
+    'depends': ['mail'],
     'data': [],
     'installable': True,
     'auto_install': False

bemade_helpdesk_one_ticket_per_email/models/mail_thread.py

@@ -1,17 +1,17 @@
 from odoo import models, fields, api, _
+import logging
+
+_logger = logging.getLogger(__name__)
 
 
 class MailThread(models.AbstractModel):
     _inherit = 'mail.thread'
 
     @api.model
-    def message_route(self, message, message_dict, model=None, thread_id=None, custom_values=None):
-        res = super().message_route(message, message_dict, model, thread_id, custom_values)
-        main_route = None
-        for tuple in res:
-            if tuple[0] in ('helpdesk.team', 'helpdesk.ticket'):
-                if not main_route:
-                    main_route = tuple
-                else:
-                    res.remove(tuple)
-        return res
+    def _message_route_process(self, message, message_dict, routes):
+        helpdesk_routes = [r for r in routes if r[0] in ('helpdesk.ticket', 'helpdesk.team')]
+        if len(helpdesk_routes) > 1:
+            _logger.info("Messages contained multiple helpdesk routes. Only the first one will be used.")
+            helpdesk_routes.remove(0)
+            routes.remove(helpdesk_routes)
+        return super()._message_route_process(message, message_dict, routes)