title: Moving project to xcode 4 gives "Apple Mach-O Linker (Id) Error
body: <p>I upgraded to XCode 4 and one of my projects won't compile anymore. Has anyone had this problem? Full error stack below (slightly reformatted for legibility):</p>
  
  <pre><code>Ld /Users/stephen/Library/Developer/Xcode/DerivedData/viewer-dsbfkuoythcgojfrsktxcccyfnuj/Build/Products/Debug-iphonesimulator/viewer.app/viewer normal i386
  cd /Users/stephen/projects/Quipper/iPhone-viewer
  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/g++-4.2 \
      -arch i386 \
      -isysroot /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator4.3.sdk \
      -L/Users/stephen/Library/Developer/Xcode/DerivedData/viewer-dsbfkuoythcgojfrsktxcccyfnuj/Build/Products/Debug-iphonesimulator \
      -L/Users/stephen/projects/Quipper/iPhone-viewer/AdMob \
      "-L/Users/stephen/projects/Quipper/iPhone-viewer/Twitter+OAuth/Libraries &amp; Headers" \
      -L/Users/stephen/projects/Quipper/iPhone-viewer/FlurryLib \
      -F/Users/stephen/Library/Developer/Xcode/DerivedData/viewer-dsbfkuoythcgojfrsktxcccyfnuj/Build/Products/Debug-iphonesimulator \
      -filelist /Users/stephen/Library/Developer/Xcode/DerivedData/viewer-dsbfkuoythcgojfrsktxcccyfnuj/Build/Intermediates/viewer.build/Debug-iphonesimulator/viewer.build/Objects-normal/i386/viewer.LinkFileList \
      -mmacosx-version-min=10.6 -Xlinker -objc_abi_version -Xlinker 2 \
      -framework CoreGraphics -framework CoreData -lAdMob -framework iAd \
      -framework StoreKit -framework MediaPlayer -lOAuth -lxml2.2 \
      -framework MessageUI -framework CoreLocation -framework QuartzCore \
      -framework CoreMedia -framework AVFoundation -framework SystemConfiguration \
      -framework CoreAudio -framework AudioToolbox -framework Foundation \
      -framework UIKit -lFlurry -framework OpenAL \
      -o /Users/stephen/Library/Developer/Xcode/DerivedData/viewer-dsbfkuoythcgojfrsktxcccyfnuj/Build/Products/Debug-iphonesimulator/viewer.app/viewer
  
  ld: duplicate symbol _OBJC_IVAR_$_StartChallengeViewController.user_pack_id
      in /Users/stephen/Library/Developer/Xcode/DerivedData/viewer-dsbfkuoythcgojfrsktxcccyfnuj/Build/Intermediates/viewer.build/Debug-iphonesimulator/viewer.build/Objects-normal/i386/StartChallengeViewController-C52C140DA244D284.o
      and /Users/stephen/Library/Developer/Xcode/DerivedData/viewer-dsbfkuoythcgojfrsktxcccyfnuj/Build/Intermediates/viewer.build/Debug-iphonesimulator/viewer.build/Objects-normal/i386/StartChallengeViewController-469DDEB33F89F484.o
      for architecture i386
  collect2: ld returned 1 exit status
  Command /Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin/g++-4.2 failed with exit code 1
  </code></pre>