Hi there. Xûr will give you duplicates if you have already unlocked all the exotics you can get from that source (random world drops). Every other exotic you're missing would be one for a different class, or one that's tied to Season Passes, the Monument to Lost Lights kiosk, quests, raids, the Shadowkeep campaign, or legend and master Lost Sectors.
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Thank you that helps A LOT! Now I know not to purchase those engrams.