old_contents: # Copyright 2021 Tecnativa- Víctor Martínez
  # License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html).
  
  from lxml import etree
  
  from odoo import api, fields, models
  
  
  class AccountJournal(models.Model):
      _inherit = "account.journal"
  
      thirdparty_invoice = fields.Boolean(string="Third-party invoice", copy=False)
  
      @api.model
      def fields_view_get(
          self, view_id=None, view_type="form", toolbar=False, submenu=False
      ):
          """Thirdparty fields are added to the form view only if they don't exist
          previously (l10n_es_aeat_sii_oca addon also has the same field names).
          """
          res = super().fields_view_get(
              view_id=view_id,
              view_type=view_type,
              toolbar=toolbar,
              submenu=submenu,
          )
          if view_type == "form":
              doc = etree.XML(res["arch"])
              node = doc.xpath("//field[@name='thirdparty_invoice']")
              if node:
                  return res
              for node in doc.xpath("//field[@name='type']"):
                  elem = etree.Element("field", {"name": "thirdparty_invoice"})
                  node.addnext(elem)
              res["arch"] = etree.tostring(doc)
              xarch, xfields = self.env["ir.ui.view"].postprocess_and_fields(
                  self._name, etree.fromstring(res["arch"]), view_id
              )
              res["arch"] = xarch
              res["fields"] = xfields
          return res
  
new_contents: # Copyright 2021 Tecnativa- Víctor Martínez
  # License AGPL-3.0 or later (https://www.gnu.org/licenses/agpl.html).
  
  from lxml import etree
  
  from odoo import api, fields, models
  
  
  class AccountJournal(models.Model):
      _inherit = "account.journal"
  
      thirdparty_invoice = fields.Boolean(string="Third-party invoice", copy=False)
  
      @api.model
      def fields_view_get(
          self, view_id=None, view_type="form", toolbar=False, submenu=False
      ):
          """Thirdparty fields are added to the form view only if they don't exist
          previously (l10n_es_aeat_sii_oca addon also has the same field names).
          """
          res = super().fields_view_get(
              view_id=view_id,
              view_type=view_type,
              toolbar=toolbar,
              submenu=submenu,
          )
          if view_type == "form":
              doc = etree.XML(res["arch"])
              node = doc.xpath("//field[@name='thirdparty_invoice']")
              if node:
                  return res
              for node in doc.xpath("//field[@name='type']"):
                  elem = etree.Element("field", {"name": "thirdparty_invoice"})
                  node.addnext(elem)
              res["arch"] = etree.tostring(doc)
              xarch, xfields = self.env["ir.ui.view"].postprocess_and_fields(
                  etree.fromstring(res["arch"]), self._name
              )
              res["arch"] = xarch
              res["fields"] = xfields
          return res
  
subject: Fix fields_view_get() function.