diff --git a/addons/account/tests/test_account_move_send.py b/addons/account/tests/test_account_move_send.py
index 7b3f7493b0a..9174deb39c3 100644
--- a/addons/account/tests/test_account_move_send.py
+++ b/addons/account/tests/test_account_move_send.py
@@ -527,6 +527,15 @@ class TestAccountMoveSend(TestAccountMoveSendCommon):
         ])
         self.assertEqual(len(invoice_attachments), 1)
 
+        # Only one PDF linked to the invoice.
+        invoice_attachments = self.env['ir.attachment'].search([
+            ('name', '=', pdf_report.name),
+            ('res_model', '=', invoice._name),
+            ('res_id', '=', invoice.id),
+            ('res_field', '=', False),
+        ])
+        self.assertFalse(invoice_attachments)
+
     def test_invoice_multi(self):
         invoice1 = self.init_invoice("out_invoice", partner=self.partner_a, amounts=[1000], post=True)
         invoice2 = self.init_invoice("out_invoice", partner=self.partner_b, amounts=[1000], post=True)
@@ -653,13 +662,20 @@ class TestAccountMoveSend(TestAccountMoveSendCommon):
         # Send.
         wizard.action_send_and_print()
         message = self._get_mail_message(invoice)
-        self.assertRecordValues(message, [{
-            'attachment_ids': [
-                manual_attachment.id,
-                invoice.invoice_pdf_report_id.id,
-                extra_attachment2.id,
-            ],
-        }])
+        self.assertRecordValues(message.attachment_ids.sorted('name'), [
+            {
+                'name': invoice.invoice_pdf_report_id.name,
+                'datas': invoice.invoice_pdf_report_id.datas,
+            },
+            {
+                'name': extra_attachment2.name,
+                'datas': extra_attachment2.datas,
+            },
+            {
+                'name': manual_attachment.name,
+                'datas': manual_attachment.datas,
+            },
+        ])
 
         # Resend.
         wizard = self.create_send_and_print(invoice)
@@ -679,12 +695,31 @@ class TestAccountMoveSend(TestAccountMoveSendCommon):
         # Send.
         wizard.action_send_and_print()
         message = self._get_mail_message(invoice)
-        self.assertRecordValues(message, [{
-            'attachment_ids': [
-                invoice.invoice_pdf_report_id.id,
-                extra_attachment2.id,
-            ],
-        }])
+        self.assertRecordValues(message.attachment_ids.sorted('name'), [
+            {
+                'name': invoice.invoice_pdf_report_id.name,
+                'datas': invoice.invoice_pdf_report_id.datas,
+            },
+            {
+                'name': extra_attachment2.name,
+                'datas': extra_attachment2.datas,
+            },
+        ])
+
+        # Manually remove the attachment and check the mail's attachments are not removed.
+        invoice_pdf_report_name = invoice.invoice_pdf_report_id.name
+        invoice_pdf_report_datas = invoice.invoice_pdf_report_id.datas
+        invoice.invoice_pdf_report_id.unlink()
+        self.assertRecordValues(message.attachment_ids.sorted('name'), [
+            {
+                'name': invoice_pdf_report_name,
+                'datas': invoice_pdf_report_datas,
+            },
+            {
+                'name': extra_attachment2.name,
+                'datas': extra_attachment2.datas,
+            },
+        ])
 
     def test_proforma_pdf(self):
         invoice = self.init_invoice("out_invoice", amounts=[1000], post=True)
diff --git a/addons/account/wizard/account_move_send.py b/addons/account/wizard/account_move_send.py
index f980135edf6..cdfa66e9bf6 100644
--- a/addons/account/wizard/account_move_send.py
+++ b/addons/account/wizard/account_move_send.py
@@ -404,7 +404,7 @@ class AccountMoveSend(models.Model):
         partner_ids = kwargs.get('partner_ids', [])
         mail_template = self.mail_template_id
 
-        move\
+        new_message = move\
             .with_context(
                 no_new_invoice=True,
                 mail_notify_author=self.env.user.partner_id.id in partner_ids,
@@ -421,6 +421,12 @@ class AccountMoveSend(models.Model):
                 },
             )
 
+        # Prevent duplicated attachments linked to the invoice.
+        new_message.attachment_ids.write({
+            'res_model': new_message._name,
+            'res_id': new_message.id,
+        })
+
     def _get_mail_params(self, move):
         self.ensure_one()
 
@@ -429,17 +435,25 @@ class AccountMoveSend(models.Model):
 
         # We must ensure the newly created PDF are added. At this point, the PDF has been generated but not added
         # to 'mail_attachments_widget'.
-        attachment_ids = list(set([
-            x['id']
-            for x in self.mail_attachments_widget + self._get_invoice_extra_attachments_data(self.move_ids)
-            if not x.get('placeholder')
-        ]))
+        seen_attachment_ids = set()
+        for attachment_data in self._get_invoice_extra_attachments_data(self.move_ids) + self.mail_attachments_widget:
+            try:
+                attachment_id = int(attachment_data['id'])
+            except ValueError:
+                continue
+
+            seen_attachment_ids.add(attachment_id)
+
+        mail_attachments = [
+            (attachment.name, attachment.raw)
+            for attachment in self.env['ir.attachment'].browse(list(seen_attachment_ids))
+        ]
 
         return {
             'body': self.mail_body,
             'subject': self.mail_subject,
             'partner_ids': self.mail_partner_ids.ids,
-            'attachment_ids': attachment_ids,
+            'attachments': mail_attachments,
         }
 
     def _send_mails(self, moves_data):
@@ -454,7 +468,8 @@ class AccountMoveSend(models.Model):
                 continue
 
             if move_data.get('proforma_pdf_attachment'):
-                mail_params['attachment_ids'].append(move_data['proforma_pdf_attachment'].id)
+                attachment = move_data['proforma_pdf_attachment']
+                mail_params['attachments'].append((attachment.name, attachment.raw))
 
             mail_lang = form.mail_lang
             email_from = form._get_mail_default_field_value_from_template(mail_template, mail_lang, move, 'email_from')