Hemochromatosis: is it possible to inherit both mutated genes from one parent, or is it always one from each parent?

One per parent. It's exceptional to inherit two alleles at one locus from a single parent, though there's some exceptional cases. Here's a heads-up, though -- hemochromatosis can occur in someone with just one dose if he/she takes in a lot of iron for some reason. Of all the highly lethal common diseases, hemochromatosis is the easiest by far to manage.