What prevents an attacker from copying and pasting a Chrome packaged app (which is just a zipped web app) then distributing it on the Chrome webstore under a slightly different name?
Just like piracy, you can't really stop it. Use legal means like the DMCA and C&D letters to stop the other person, or report it to Google once you find out.
Google might have features that check the hash of an app to see if it's a copy of another person's app, but they might not. Understand that there's nothing that you can do if you don't know a fradulent app exists - if you find one than do whatever it takes to get his app off the market.