Hi!
I see some people mentioning getting duplicates from Xur's fated engram. I'm not sure if this is a bug or not, but whatever it may be I have a small tip that might help. (:
If you right click the fated engram before buying it from Xur, you'll get a screen showing six different exotics you can get from that engram. This list changes every few minutes! If you wait until the exotic you'd like shows up, you'll have a much better chance of getting it. (:
Cheers!
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That list changes because the loot pool isn’t exclusive to those 6 items. Basically the preview is a preview of what you can get.