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Not to vouch, but he could have gotten it from Emerald, couldn't he? Save in front of the legit Deoxys, then keep trying until it's shiny. Not saying that one is, but it is possible.
also, if you can wait until later tonight i am getting lots of new ones that are listed in my section of trades that i still have to conduct on the bottom of my list
yoshimitsu how do you know its fake? ichida is right i think, there were US events where you could get to a point of standing right in front of a deoxys and save