So, in the Soldier career, both the Heavy and Commando trees have Armor Master. Now, this is not a ranked talent, so the question is this:

If I buy Armor Master in Commando, do I also need to purchase it in Heavy to get the Heavy Hitter talent? Or because I have it already, can I just skip straight from Dedication to Heavy Hitter?

The latter. Enjoy.

Any time you reach a non-ranked talent in a tree that you already have, you just tick it off. The only reason not to tick them off as soon as you acquire the specialisation is to make sure you don't accidentally take new talents you haven't actually unlocked yet.