old_contents: # Copyright 2020 Tecnativa - Víctor Martínez
  # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
  
  from odoo import models
  
  
  class BasePartnerMergeAutomaticWizard(models.TransientModel):
      _inherit = "base.partner.merge.automatic.wizard"
  
      def _merge(self, partner_ids, dst_partner=None, extra_checks=True):
          if dst_partner:
              contacts = self.env["mailing.contact"].search(
                  [("partner_id", "in", partner_ids)]
              )
              if contacts:
                  contacts = contacts.sorted(
                      lambda x: 1 if x.partner_id == dst_partner else 0
                  )
                  list_ids = contacts.mapped("list_ids").ids
                  contacts[1:].unlink()
                  contacts[0].partner_id = dst_partner
                  contacts[0].list_ids = [(4, x) for x in list_ids]
          return super()._merge(
              partner_ids, dst_partner=dst_partner, extra_checks=extra_checks
          )
  
new_contents: # Copyright 2020 Tecnativa - Víctor Martínez
  # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
  
  from odoo import models
  
  
  class BasePartnerMergeAutomaticWizard(models.TransientModel):
      _inherit = "base.partner.merge.automatic.wizard"
  
      def _merge(self, partner_ids, dst_partner=None, extra_checks=True):
          if dst_partner:
              contacts = (
                  self.env["mailing.contact"]
                  .sudo()
                  .search([("partner_id", "in", partner_ids)])
              )
              if contacts:
                  contacts = contacts.sorted(
                      lambda x: 1 if x.partner_id == dst_partner else 0
                  )
                  list_ids = contacts.mapped("list_ids").ids
                  contacts[1:].unlink()
                  contacts[0].partner_id = dst_partner
                  contacts[0].list_ids = [(4, x) for x in list_ids]
          return super()._merge(
              partner_ids, dst_partner=dst_partner, extra_checks=extra_checks
          )
  
subject: Add sudo() to prevent user without mailing access try to merge contacts