title: Error: duplicate symbols for architecture armv7 after change Core Data Model
body: <p>I have an app using core data framework. I was working fine. I just changed the data model - add an attribute to one entity.
  And when I try to build it, I got an error:</p>
  
  <pre><code>duplicate symbol _OBJC_METACLASS_$_AccountFolder in:
  /Users/XXX/Library/Developer/Xcode/DerivedData/MyApp-bxsswgxdenxgjweotkkkckaoalat/Build/Intermediates/MyApp.build/Debug-iphoneos/MyApp.build/Objects-normal/armv7/AccountFolder-33D7EA63E98D6090.o
  
  ld: 4 duplicate symbols for architecture armv7
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  </code></pre>
  
  <p>I did some search, and most people getting this error is from mistyping .h to .m, or import same .h several times. I checked my code. I don't have that.
  And the coredata framework is still there.</p>
  
  <p>Does anyone know what else can be the reason?
  Thanks.</p>