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Maybe depending on the exchange it could have been an activity bonus?

I don't think so. The exact amount sent twice says it is not a bonus. Plus, they sent no email about it to him.
And most importantly, his records inside the exchange does not show the extra transfer! It is definitely a mistake.

Well I would alert them, something like that could be the end of the exchange. As to giving back, alert as a third party for internal review purposes. They messed up and unless they put some lean on his account which if you only say you know it has happened it should all be good.

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