You're allowed to swap either weapon or both weapons for anything on the list, even doubles. You're allowed to be armed with both a Storm Shield and Thunder Hammer, you're allowed to be armed with two power weapons, and you're allowed to be armed with two storm bolters.
"Trying to make some sense of it all,
When I can see it makes no sense at all"
"Fools to the left of me,
Jokers to the right,
Here I am,
Stuck in the middle with you."