I would also post an alert just in case he tries this with someone who didn't check Sandbox... This is one people need to know about.
The real thatdude is a great guy, this other asshat is either LE or looking to fuck with people. Please be sure to include the number he contacted you with, and any email addresses he's contacted you with. And if he has a number you don't give out, and you haven't slipped up anywhere along the lines with that, then it sounds like someone you know is stirring up shit with you.
Excellent report though, I hope everything turns out ok...