Add to /Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Versions/A/Resources/IDETextKeyBindingSet.plist
<key>Duplication</key>
<dict>
<key>Duplicate Current Lines Down</key>
<string>selectParagraph:, delete:, yank:, moveToBeginningOfParagraph:, yank:, moveUp:, moveToEndOfParagraph:</string>
Delete Current Line