title: XCode: error: linker command failed with exit code 1 (use -v to see invocation)
body: <p>I am working on a project, and when I build the project, I got an issue which is the following:</p>
  
  <pre><code>Ld /Users/KiKo-SaMa/Library/Developer/Xcode/DerivedData/SpaceInvaders-cgcsumybmrxbddfqdwhevdvubdic/Build/Products/Debug/SpaceInvaders.app/Contents/MacOS/SpaceInvaders normal x86_64
      cd /Users/KiKo-SaMa/Desktop/SpaceInvaders
      setenv MACOSX_DEPLOYMENT_TARGET 10.6
      /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -arch x86_64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.8.sdk -L/Users/KiKo-SaMa/Library/Developer/Xcode/DerivedData/SpaceInvaders-cgcsumybmrxbddfqdwhevdvubdic/Build/Products/Debug -F/Users/KiKo-SaMa/Library/Developer/Xcode/DerivedData/SpaceInvaders-cgcsumybmrxbddfqdwhevdvubdic/Build/Products/Debug -filelist /Users/KiKo-SaMa/Library/Developer/Xcode/DerivedData/SpaceInvaders-cgcsumybmrxbddfqdwhevdvubdic/Build/Intermediates/SpaceInvaders.build/Debug/SpaceInvaders.build/Objects-normal/x86_64/SpaceInvaders.LinkFileList -mmacosx-version-min=10.6 -fobjc-arc -fobjc-link-runtime -framework Cocoa -o /Users/KiKo-SaMa/Library/Developer/Xcode/DerivedData/SpaceInvaders-cgcsumybmrxbddfqdwhevdvubdic/Build/Products/Debug/SpaceInvaders.app/Contents/MacOS/SpaceInvaders
  
  duplicate symbol _Cycles in:
      /Users/KiKo-SaMa/Library/Developer/Xcode/DerivedData/SpaceInvaders-cgcsumybmrxbddfqdwhevdvubdic/Build/Intermediates/SpaceInvaders.build/Debug/SpaceInvaders.build/Objects-normal/x86_64/emulator.o
      /Users/KiKo-SaMa/Library/Developer/Xcode/DerivedData/SpaceInvaders-cgcsumybmrxbddfqdwhevdvubdic/Build/Intermediates/SpaceInvaders.build/Debug/SpaceInvaders.build/Objects-normal/x86_64/SpaceInvadersMachine.o
  ld: 1 duplicate symbol for architecture x86_64
  clang: error: linker command failed with exit code 1 (use -v to see invocation)
  </code></pre>
  
  <p>At first, I looked at the questions here and tried to understand the error message, it seemed I had a duplicate variable name, so I changed it and when I try to build it again, I keep getting the same error, I checked also for duplicate function names, nothing seems to be wrong with my code. I also checked for the includes, because in a question, one of the solutions was including .m instead of .h. Can someone please help me with this.</p>
  
  <p>I don't know if this is relevant, but at first I got 3 issues, it seemed I had to add to the "Compile Sources" of the "Build Phases" of my project two C files, and now I only get 1 issue which is the above one.</p>
  
  <p>Thanks</p>