One will always be the original copy, not the transported "clone" in this case. So whichever one didn't pop out of the transporter after their matter was combined on the pad, and therefore isn't 10 seconds old, is the "real" you.

Although I would still argue that the cloned copy is ALSO you, it's still not the original.

Of course all you have to do is ask Miles Obrien and he will tell you dark secrets starfleet doesn't want anyone in the federation to know. Transporters kill the original to prevent this scenario from happening, and only on rare occasions does it fail to delete someone, like RIKER.OG.