New small module to have only one ticket created per email received.

bemade_helpdesk_one_ticket_per_email/__init__.py

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+from . import models

bemade_helpdesk_one_ticket_per_email/__manifest__.py

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+#
+#    Bemade Inc.
+#
+#    Copyright (C) 2023-June Bemade Inc. (<https://www.bemade.org>).
+#    Author: Marc Durepos (Contact : mdurepos@durpro.com)
+#
+#    This program is under the terms of the Odoo Proprietary License v1.0 (OPL-1)
+#    It is forbidden to publish, distribute, sublicense, or sell copies of the Software
+#    or modified copies of the Software.
+#
+#    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+#    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+#    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
+#    IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+#    DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+#    ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+#    DEALINGS IN THE SOFTWARE.
+#
+{
+    'name': 'Helpdesk One Ticket Per Email',
+    'version': '15.0.1.0.0',
+    'summary': 'Restrict ticket creation to a single ticket per email received.',
+    'category': 'Helpdesk',
+    'author': 'Bemade Inc.',
+    'website': 'http://www.bemade.org',
+    'license': 'OPL-1',
+    'depends': ['helpdesk'],
+    'data': [],
+    'installable': True,
+    'auto_install': False
+}

bemade_helpdesk_one_ticket_per_email/models/__init__.py

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+from . import mail_thread

bemade_helpdesk_one_ticket_per_email/models/mail_thread.py

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+from odoo import models, fields, api, _
+
+
+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