I think this game increases odds of you receiving a weapon if you've already gotten it. I swear it. I had two legendary heavy engrams this weekend. One just sitting at Postmaster (must've missed it), sharded. I then level'd cryptarch to find that I got another heavy legendary engram. I'm like "Okay this time it won't shard me. Come on Gjally." Boom exotic...Truth....my 7th. I literally only ever get Truth or Thunderlord (4 of them). Dragons Breath dropped once.