CodeSlicing

    <key>User Defined</key>
    <dict>      
        <key>Insert New Line Above Current Line</key>
        <string>moveUp:, moveToEndOfLine:, insertNewline:</string>
        <key>Insert New Line Below Current Line</key>
        <string>moveToEndOfLine:, insertNewline:</string>
        <key>Duplicate Current Lines Down</key>
        <string>selectParagraph:, delete:, yank:, moveToBeginningOfParagraph:, yank:, moveUp:, moveToEndOfParagraph:</string>
    </dict>

emotality

NOTE (2022-07-09): Xcode finally added this functionality in Xcode 14, please see release notes here:

New Features in Xcode 14 Beta 3
When editing code, the Edit > Duplicate menu item and its corresponding keyboard shortcut now duplicate the selected text — or the line that currently contains the insertion point, if no text is selected. (8614499) (FB5618491)

---

Xcode line duplicate

Bind keys to duplicate lines in Xcode

artifactsauce

#!/bin/sh

CMDNAME=`basename $0`

while getopts n OPT
do
  case $OPT in
    "n" ) DRY_RUN="--dry-run" ;;
      * ) echo "Usage: $CMDNAME [-n]" 1>&2
          exit 1 ;;