Character Card System

I'm a little bit confused on how doubling up works in these scenarios.

Since I'm going to be designing my character cards around using an Aran, I'm almost positive I want one of my sets to be Paladin/Dark Knight/Demon Slayer, but are you telling me that if I make two of those that I won't get all the stacking bonuses?

So in order of preference
1. Pally/DrK/DS x2 (if everything stacks)
2. Pally/DrK/DS, Pally/DrK/Mech (if individual class bonuses stack but set bonuses don't)
3. Pally/DrK/DS, Marksman/BaM/Mech (if no duplicates stack)

I know I worded that a little awkwardly, but I hope I got my point across and hopefully someone can tell me how these interact together before I level up a billion different things. Also, is the paladin buff actually attack or is it just damage? Because +300 or +400 attack for a level 200 sounds ridiculous.

Individual class bonuses stack (eg. a Paladin in both sets), class-set bonuses are only 1 at a time (it prefers the higher level one).

The Paladin buff is adding the character's current level (times whatever) to the damage range.