Hi,

Try this code:

from odoo import models,api
from odoo.exceptions import ValidationError


class AccountMove(models.Model):
    _inherit = 'account.move'

    @api.model
    def create(self, vals):
        credit = self.env['account.move'].search(
            [('reversed_entry_id', '=', self.id)])
        if credit and self.id:
            raise ValidationError(
                "Cannot create a credit note for an already refunded invoice.")
        return super(AccountMove, self).create(vals)

Hope it helps