So I just bought the special engram from Xur. I have many exotics still to collect, just from the ones that I can see on collections that aren't blanked out. Yet I got a duplicate. I was given Prometheus lens, which I acquired for the first time when Curse of Osiris was released. I wonder if this was a glitch, or perhaps there are only certain ways to acquire the forsaken exotics like quests that I have not found? Either way, some clarification would be helpful.