So, we've had two Dragon TV events, where you collect 50 coins (25 days) and get a chest with reward dragons in them. I've gotten same same reward dragon from both chests. And it's the only dragon that I previously own from the reward list.
It's so unlikely to happen, that I'm suspecting it's not accidental. It seems that the game developers want me to get a worthless reward for watching ads for 25 days, so that I will keep doing it more and more, in hopes of a better reward.
It's near impossible to get the same dragon twice. I don't believe that it's just bad luck.