i don't know if it is just me but Xur has given me a few duplicates now despite the exotics being in my collection already. It's frustrating wasting so many legendary shards on an engram which specifies that it doesn't give duplicates, just to get a duplicate.
PLEASE FIX.
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You get duplicates if he can't give you anything you don't have. Xur won't give you any exotics that come from quests, or from content you do not own.