http://stackoverflow.com/questions/551383/xcode-duplicate-delete-line
http://stackoverflow.com/questions/10266170/xcode-4-duplicate-line
适用于最新的Xcode 4.5.1
运行：
sudo chmod 666 /Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Resources/IDETextKeyBindingSet.plist
然后打开/Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Resources/IDETextKeyBindingSet.plist，在Deletions下面增加：Delete Current Line，值为：
deleteToBeginningOfLine:, moveToEndOfLine:, deleteToBeginningOfLine:, deleteBackward:, moveDown:, moveToBeginningOfLine:
在Insertions and Indentations下面增加：Duplicate Current Line，值为：
selectLine:, copy:, moveToEndOfLine:, insertNewline:, paste:, deleteBackward:
然后重启Xcode，打开设置，Key Bindings，找到“Delete Current Line”和”Duplicate Current Line”这两项，设置为自己想要的快捷键。