I don't see a problem with having multiple offers out. Like Gom said, the software cancels out the other offer once one is accepted. In other words, the first deal struck is the deal. I don't see how that is any different than talking to more than one person via PM, AIM, other form of media. In those forms, the first deal struck wins too.
Gom, I'm looking at your offer and I don't like the fit. I'm not going to outright reject it just yet, but if I do counter, expect something a little different.