Duplicate Current Line - Append to /Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Resources/IDETextKeyBindingSet.plist
<key>Custom Commands</key>
<dict>
<key>Duplicate Current Line</key>
<string>selectLine:, copy:, moveToEndOfLine:, insertNewline:, paste:, deleteBackward:</string>
</dict>