Confession: I have done this.
I don't think there's any way to really know when a borrower is doing such a thing, because of the credit bureau reporting lag. I'm far more of an investor than a borrower and so I would not be opposed to either 1) an industry-wide overhaul of the standard bureau reporting process or 2) some sort of cross-reporting agreement between Prosper and Lending Club to notify investors on the other platform of these "simultaneous borrowers". Otherwise I don't think there is really any way to identify situations like this.
Although now that you have me thinking about it, it probably wouldn't be too difficult to design a mechanism that checks for listings with similar credit profiles on the opposite platform. I'll look into doing this if it's something people would find useful...
Also as far as I know, once the money hits the borrower's bank account, as long as there is no fraud or other legal question surrounding the loan, it can't be reversed.