AWB does not deal with a Module being required more than once

If a Module is required by more than one other Module in the Model hierarchy, the configure script fails:

0x605c68Can't find required module 'x' in model 'y'
Error: incompletly instantiated model Unamed project
       Can't create build tree

However, the GUI deals with it just fine. 

This should be a problem only if a different Module implementation is selected at each location.
