Many traits grant a new type of bonus: a “trait” bonus. Trait bonuses do not stack—they’re intended to give you a slight edge over the non-PCs of the world, not a secret backdoor way to focus all your traits on one type of bonus to gain an unseemly advantage over the rules. It’s certainly possible, for example, that somewhere down the line, a “Courageous” trait might be on the list of dwarf race traits, but just because this trait’s on the dwarf race traits list and the basic combat traits list doesn’t mean you’re any more brave if you choose both than if you choose only one.