Try checking out the version of Xcode you are using. In the terminal, run the following command
xcode-select -print-path
then try checking out this version of Xcode 4.6.3 or lower if you don't change the path
sudo xcode-select -switch /Applications/Xcode.app/Contents/Developer
where Xcode.app is Xcode 4.6.3 or lower The version path.
after switching the xcode version try downloading it may solve your problem
if you have a beta version of Xcode 5 then it gives a warning
silentBeep
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