Answer 1

1. Go to this folder which contains dark side of the force:

   • Xcode 4.2 or prior: /Developer/Library/PrivateFrameworks/IDEKit.framework/Resources
   • Xcode 4.3 or later: /Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Resources.

2. Open IDETextKeyBindingSet.plist.

3. Add a new dictionary and a new command item as the screenshot below (name them what you want): 

   That's: selectLine:, copy:, moveToEndOfLine:, insertNewline:, paste:, deleteBackward:

4. Restart Xcode and go to Preferences - Key Bindings, search for your command.

5. Set a key combination for the command: 

6. Finally unleashed the power of key bindings on Xcode... Enjoy it!

Answer 2

Update 2020

Works on Xcode versions: 11.3.1 too

Works in Xcode versions: 7.1, 7.3, 7.3.1, 8.0, 8.1, 8.2, 8.3, 9.0, 9.1, 9.2, 10.0, 10.1, 10.2.1

Instruction

1. Give permissions to edit the following files. Most of the time OS will not allow you to edit the files.

for 11.3.1 version

sudo chmod 666 /Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Versions/A/Resources//IDETextKeyBindingSet.plist

sudo chmod 777 /Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Versions/A/Resources/

below 11.3 versions

sudo chmod 666 /Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Resources/IDETextKeyBindingSet.plist

sudo chmod 777 /Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Resources/

2. Edit the the file, by writing the following command: (Open it in sublime or text editor if you can)

for 11.3.1

open  

/Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Versions/A/Resources/

for below versions

open /Applications/Xcode.app/Contents/Frameworks/IDEKit.framework/Versions/A/Resources/IDETextKeyBindingSet.plist

3. Append the following xml snippets:

<key>custom</key>
<dict>
    <key>Duplicate Current Line</key>
    <string>moveToBeginningOfLine:, deleteToEndOfLine:, yank:, insertNewline:, moveToBeginningOfLine:, yank:</string>
    <key>Duplicate Lines</key>
    <string>selectLine:, copy:, moveToEndOfLine:, insertNewline:, paste:, deleteBackward:</string>
    <key>Delete Line</key>
    <string>selectLine:, deleteBackward:</string>
</dict>

---

Screenshots

and Restart Xcode and go to Preferences - Key Bindings, search for the command we typed (Duplicate Lines&Duplicate Current Line).

Thats it.