This week I got 2 duplicate exotics as powerful gear rewards, Sunshot two days ago and Crimson today, both of which I already had. I thought when we got an exotic it was guaranteed not to be something we already had - am I mistaken about that or is this a bug introduced with the update on Tuesday? I have most of the exotics in the game and this has never happened before Tuesday’s update. Thanks.
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There has never been any kind of protection to stop you from getting exotics you already have. There was protection added in year 1 to prevent getting the same exotics as back-to-back powerful drops, and exotic drops are weighted towards something you don't already have, but there is no guarantee you won't get a repeat, especially if you already have most of the exotics. Xur's Fated Engram is the only way to get a guaranteed non-repeat exotic, within its own rules/restrictions.