Hey there. If an exotic engram drops a duplicate, then it means you already have all 'world drop' exotics. The two exotics needed on your Titan, three on your Hunter and four on your Warlock, only drop from Legend/Master Lost Sectors, they cannot drop initially from an Exotic Engram. (The above applies to engrams only, some sources such as Ordeals and also the Legend/Master Lost Sectors which do not have duplicate protection) Similarly all the weapons you need are quest/kiosk ones, all world drops have been unlocked.