Nothing to do with Duty, Harvey's may pay slightly more duty on their products than Long Man because they are a larger brewery (but still pretty small in the grand scheme of things) but not enough to account for a 50% price hike. Last time I looked at prices for various Sussex breweries Harvey's was the most expensive by a considerable margin. Harvey's Best is the most expensive 4.0% Best Bitter on the market, as far as I know. Many of it's direct competitors London Pride, Adnams Southwold; Young's Best and Doom Bar are considerably cheaper. Landlords cannot get away with just charging £5 a pint for Harvey's though.
As to Wetherspoons that is simple economies of scale. They will buy an entire months worth of production off those smaller breweries. Why is Wetherspoons cheaper than a standard pub? Why is Tesco cheaper than your local corner shop?
Can't talk about Wetherspoon but I do know that DrinkInBrighton used to stock Harvey's until they jacked the prices up to an unaffordable rate (thinking they were irreplaceable in Sussex). To combat it they set up the North Laine Brewery and did their own Best Bitter for considerably less cost whilst still charging similar.