Error running app : 'xxx multiply defined'

What should one do to build blackberry apps for different OS versions? I've seen strange behavior when th开发者_开发百科e same app runs well at 9700 OS 5.0, 9800 6.0 but gives an error at 9700 OS 6.0:

Error Starting jokes_480x360_: Class 'com.core.controller.LastMessages' multiply defined

---

A version of your application is already installed.
Remove it before installing.
Refer to this article in Blackberry Knowledge base