I've heard that once there was a glitch where if you had any exotic equiped that you were more likely to get something you already had. Either they fixed that or I've been lucky. Only one double so far. If you find that many exotics try de-crypting one then do another activity then decrypt another. I've found even in D1 that spamming exotic engrams can go against you. At least your getting armor its more versatile than getting guns. For instance my friend has gotten merciless 6 times and because you can only unfuse into other fusion rifle the exotics end up being shards instead of infusing other gun types at least you can infuse any leg armor on that character you want. To add to this it's confirmed that if you decrypt on a hunter lets say you will not get a titan armor piece. etc.