I got a message from someone regarding this thread today.  In the spirit of transparency, instead of privately answering them, i'm going to publicly post a response.

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b) Someone deposited funds to our exchange and tried to social engineer double crediting of a single deposit of coins.  This is not a flaw with NEM;  If an exchange isn't properly tracking txids, someone can try to double claim something deposited.  We have multiple layers to safeguard against that.

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I hope this answers the query we got and sheds light on what happened from our side.  

Thanks,
richie@bittrex