title: build failed with: ld: duplicate symbol _OBJC_CLASS_$_Algebra5FirstViewController
body: <p>I am getting this error suddenly when running my app via iPhone simulator:</p>
  
  <blockquote>
    <p>clang: error: linker command failed with exit code 1 (use -v to see
    invocation):</p>
    
    <p>ld: duplicate symbol _OBJC_CLASS_$_Algebra5FirstViewController in
    .../Algebra5-anwcuftojtxtgkfootneeeqhwztj/Build/Intermediates/Algebra5.build/Debug-iphonesimulator/Algebra5.build/Objects-normal/i386/ExercisesViewController.o
    and 
    .../Algebra5-anwcuftojtxtgkfootneeeqhwztj/Build/Intermediates/Algebra5.build/Debug-iphonesimulator/PSLE
    Algebra5.build/Objects-normal/i386/PSLE_Algebra5FirstViewController.o
    for architecture i386</p>
  </blockquote>
  
  <p>What is it all about?</p>