I already paid LLC annual fee for 2020, but there is $800 due. Do I need to pay it again?

Yes, if you filed the LLC articles of organization in 2019, then that would be considered it's first year and owe $800.  Then, in 2020 you are beginning it's second year of business.  You would owe $800 also for 2020 using payment voucher 3522

For CA LLCs:

Penalties and Interest If the LLC fails to pay its annual tax by the 15th day of the 4th month after the beginning of the taxable year, a late payment penalty plus interest will be assessed for failure to pay the annual LLC tax by the due date.

The penalty and interest will be computed from the due date of the tax to the date of payment.

Late Payment of Prior Year Annual LLC Tax:

If a prior year annual LLC tax of $800 was not paid by the 15th day of the 4th month after the beginning of the taxable year, the tax should be paid as soon as possible, using the appropriate taxable year form FTB 3522. Do not use any other form for payment of the tax. This will assure proper application of the payment to the LLC’s account.

The annual LLC tax is due and payable by the 15th day of the 4th month after the beginning of the LLC’s taxable year (fiscal year) or April 15, 2020 (calendar year).

When the due date falls on a weekend or holiday, the deadline to file and pay without penalty is extended to the next business day.

The first taxable year of an LLC begins when the LLC files its articles of organization with the SOS.

The first taxable year of a foreign LLC doing business in California begins when the LLC is organized in another state.

If the 15th day of the 4th month of an existing foreign LLC’s taxable year has passed before the foreign LLC commences business in California or registers with the SOS, the annual LLC tax should be paid immediately after commencing business or registering with the SOS.

For example, if you filed the LLC articles of organization with the SOS in November 2019, then you will owe $800 for 2019.  Then, on April 15 of 2020, you will owe another $800.