Every rare seems to have 12,5% chance of pulling. With every rare you have that chance of obtaining a ninja you don't have decreases by 12,5%. There's no hidden "make life difficult for that player in particular" meter, it all depends on luck. Ive pulled 6 rares and not a single one of them was a duplicate, while other people pull 2 rares and their 3rd is already a duplicate. Statistically after your 4th rare you have 50/50 % to get either new or duplicate, after that the chance of obtaining something new only decreases. It isnt any mathematical proof because that takes into account less than 10 players, but some of the people on my server after pulling 4 different rares usually get 2-3 duplicates before getting a brand new rare (applies both to kage treasure and summon treasure).