title: Command /Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin/clang failed with exit code 1
body: <p><img src="https://i.stack.imgur.com/rJzlz.png" alt="Error screenshot">
  Can't understand how to fix it and why it appears?</p>
  
  <pre><code>    Ld /Users/atencion/Library/Developer/Xcode/DerivedData/Flylista-gcadfttyrgznkmafzaicacwytfey/Build/Products/Debug-iphonesimulator/Flylista.app/Flylista normal i386
      cd "/Users/atencion/Documents/Applications Development/Xcode Projects/Flylista"
      setenv MACOSX_DEPLOYMENT_TARGET 10.6
      setenv PATH "/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin"
      /Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin/clang -arch i386 -isysroot /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk -L/Users/atencion/Library/Developer/Xcode/DerivedData/Flylista-gcadfttyrgznkmafzaicacwytfey/Build/Products/Debug-iphonesimulator -F/Users/atencion/Library/Developer/Xcode/DerivedData/Flylista-gcadfttyrgznkmafzaicacwytfey/Build/Products/Debug-iphonesimulator -filelist /Users/atencion/Library/Developer/Xcode/DerivedData/Flylista-gcadfttyrgznkmafzaicacwytfey/Build/Intermediates/Flylista.build/Debug-iphonesimulator/Flylista.build/Objects-normal/i386/Flylista.LinkFileList -mmacosx-version-min=10.6 -Xlinker -objc_abi_version -Xlinker 2 -fobjc-arc -Xlinker -no_implicit_dylibs -D__IPHONE_OS_VERSION_MIN_REQUIRED=50000 -framework QuartzCore -framework UIKit -framework Foundation -framework CoreGraphics -o /Users/atencion/Library/Developer/Xcode/DerivedData/Flylista-gcadfttyrgznkmafzaicacwytfey/Build/Products/Debug-iphonesimulator/Flylista.app/Flylista
  
  ld: duplicate symbol _OBJC_CLASS_$_FlightsViewController in /Users/atencion/Library/Developer/Xcode/DerivedData/Flylista-gcadfttyrgznkmafzaicacwytfey/Build/Intermediates/Flylista.build/Debug-iphonesimulator/Flylista.build/Objects-normal/i386/FlightViewController.o and /Users/atencion/Library/Developer/Xcode/DerivedData/Flylista-gcadfttyrgznkmafzaicacwytfey/Build/Intermediates/Flylista.build/Debug-iphonesimulator/Flylista.build/Objects-normal/i386/FlightsViewController.o for architecture i386
  Command /Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin/clang failed with exit code 1
  </code></pre>