title: Duplicate symbols for architecture x86_64 under Xcode
body: <p>I now have the same question with above title but have not found the right answer yet. I got the error:</p>
  
  <pre><code>    /Users/nle/Library/Developer/Xcode/DerivedData/TestMoboSDK-Client-cgodalyxmwqzynaxfbbewrooymnq/Build/Intermediates/TestMoboSDK-Client.build/Debug-iphonesimulator/TestMoboSDK-Client.build/Objects-normal/x86_64/MoboSDK.o
      /Users/nle/Library/Developer/Xcode/DerivedData/TestMoboSDK-Client-cgodalyxmwqzynaxfbbewrooymnq/Build/Products/Debug-iphonesimulator/libMoboSDK.a(MoboSDK.o)
  duplicate symbol _OBJC_METACLASS_$_MoboSDK in:
      /Users/nle/Library/Developer/Xcode/DerivedData/TestMoboSDK-Client-cgodalyxmwqzynaxfbbewrooymnq/Build/Intermediates/TestMoboSDK-Client.build/Debug-iphonesimulator/TestMoboSDK-Client.build/Objects-normal/x86_64/MoboSDK.o
      /Users/nle/Library/Developer/Xcode/DerivedData/TestMoboSDK-Client-cgodalyxmwqzynaxfbbewrooymnq/Build/Products/Debug-iphonesimulator/libMoboSDK.a(MoboSDK.o)
  ld: 75 duplicate symbols for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  </code></pre>
  
  <p>Any help is appreciated.</p>
  
  <p><strong>Finally</strong> I find out the reason of this error cause I added <code>-ObjC</code> to the <code>Other Linker Flags</code>. After remove this value then I can build my project successfully, but I don't know why. Can anyone explain this?</p>