After you click the "Trade In" to get an exotic dragon. The one you will get next is random. From what we know until now.
For getting the battle dragons is different. They go from the weakest which is the Battle Monstrous Nightmare and most probably continue up to the Battle Sword Stealer. The Battle dragons for now are 20. And after you reach and get the Battle Sword Stealer. The chain starts from the beginning again most probably.
Only way to get 2 battle dragons if the same type is if you get them from special events also. Or have reached the Battle Sword Stealer and have started the chain from the beginning.
Stats wise. Concerning battle.
The only good battle dragon from all these 20 is the Battle Submaripper.
it's the best tank dragon. And you may need only 1 or 2.
what I have done: I completed the exotic tide glider collection, after trading it in, I had the exotic thunderpede collection, I completed that too… After completing the exotic thunderpede collection, The exotic tide glider showed up (again)… i completed that… now I have the exotic thunderpede collection (again).
I guess that's why it's made so much harder to get. Because it's a class of it's own compared to the normal Smothering Smokebreath. Which is 1 of the weakest dragons in the game for farming and battle.