And to clarify, you cannot choose Rifleman again even though it says you may choose whatever specialty you like, correct?
Exactly. FL doesn't really like negative rules. So they try to avoid "you can't" rules as much as they can (they try to use positive rules instead).
When it comes to specialties (or talents in other games), they represent a unique knowledge, in contrast to skills, which represent a broad knowledge.
In those FL games where you are allowed to take a specialty more than once it will say so in that specific specialty (i.e. a positive rule). In T2K4E specifically I don't think there are any specialties that you can take more than once. In other YZE games that have talents that you can take more than once they are either listed in ranks, like Axe Fighter rank 1, Axe Fighter rank 2, etc in FbL or you have to specify a field when you take them, like Weapon Specialist in Alien RPG that you can take more than once as long as you choose different weapons.