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Default bindings:
Windows: Shift + Alt + Down/Up
Mac: Shift + Option + Down/Up
Linux: Control + Shift + Alt + Down/Up
(Might need to use numpad Down and Up for Linux)
You can check the bindings at:
File > Preferences > Keyboard Shortcuts
or you can set them in "keybindings.json" via:
{
"key": "ctrl+d", // set your preferred key comb.
"command": "editor.action.duplicateSelection"
}