Why do more than one person have any specific exotic? Each one has a story behind it in the grimore, making it a unique item that in reality can only be one of a kind...that's why...
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An engram is - to quote Rahool - "Data free from matter". It's a blueprint, really. So while the pattern is unique, it seems that if you have enough matter, you can build from the blueprint. It's the engram; the "pattern" which is common, rare, legendary or exotic. Not the item itself.
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Not Ice Breaker or the Suros if I recall correctly