How long can the overpaid patent fee be refunded?

If the patent fee is paid again, the party concerned may request a refund within 3 years from the date of payment. The refund request shall be made in writing, explaining the reasons and attaching the corresponding certificates. The original remittance voucher of the post office or bank (or a notarized copy of the post office or bank) shall be provided, indicating the application number or patent number, information to be refunded (such as bill number and fee amount) and payee information. If the refund is made through the post office, the payee information includes the name, address and postal code; If the refund is made through the bank, the payee information includes the name, bank and account number. Re-payment cannot be used as the annual fee for the following year.