I got this error while working on my latest iPhone app: duplicate symbol _OBJC_METACLASS_$

Turns out that after adding an NSManagedObject subclass for a Core Data entity I moved the files into a group (to make the files look neater). When I generated them again after updating the entity it create 2 sets of the files in the structure.

Basically I deleted both instances of the files and recreated them and that fixed the issue.