class AccountInvoice(models.Model):
    _inherit = ["account.invoice"]

    duplication_number = fields.Char(string="Duplication Number")
    @api.multi
    def check_fields(self):
        print self.id
        invoices = self.env['account.invoice'].search([])
        for record in invoices:
            if record.duplication_number != self.duplication_number:
                raise UserError(_("You can't save this record, the same Vendor Bill can only be used one time."))
    @api.multi
    def write(self, vals):
        self.check_fields()
        res = super(AccountInvoice, self).write(vals)
        return res

I don't understand why my "self" in check_fields(self) is empty everytime.

I can't get the actual value of the field: duplication_number in self. The return of self is account.invoice(), everytime.

I need to get the actual value of my field "duplication_number" when I want to save it. It should check if this field has the same value as any other field in account.invoice