A 1920 Buffalo Head nickel with an "S" mint mark on the front and back of the coin is a special variety known as a "Double S" or "Mule" Buffalo nickel. These coins were produced at the San Francisco Mint in 1920, but due to a mix-up in the dies used for striking, the "S" mint mark was applied twice instead of just once.
Since these coins, while rare and not as common as the original Buffalo nickels, are still somewhat available and not incredibly rare, they usually carry moderate value with the coin alone holding a price of 7-12 dollars.