it is true that in some cases, the people simply have a metabolic abnormality that causes them to be unable to lose weight, but most people are fat because they eat poorly/gluttonously and don't exercise. I'm one of those fat people, and I don't want to pay double. Though, really, I don't take up more than one seat, I'm not THAT fat.
I dunno about double, but they should be charged an SSR (Special Service Request) Fee for needing two seats, and be given both seat assignments so nobody gets the seat next to them. SSR fees are usually for stuff like infants, needing a special dietary meal, wheelchair access, stuff like that. I don't see how obesity wouldn't fit in with those. Anyhoo, I know this crap because I program airline software. It seems reasonable to charge them an extra 50%, so that helps recoup the cost of the lost seat for the airline, but isn't as punitive to the customer.