Unity Shift-Duplicate

ShiftDuplicateEditor

using UnityEngine;
using UnityEditor;
using ProBuilder.Core;
using ProBuilder.EditorCore;

[InitializeOnLoad]
static class ShiftDuplicate
{
	static bool m_IsDragging = false;

	static ShiftDuplicate()
	{
		SceneView.onSceneGUIDelegate += OnSceneGUI;
	}

	static void OnSceneGUI(SceneView scn)
	{
		Event e = Event.current;

		if(!m_IsDragging && e.modifiers == EventModifiers.Shift && e.type == EventType.MouseDrag)
		{
			m_IsDragging = true;

			// probuilder-specific
			if(pb_Editor.instance != null && pb_Editor.instance.editLevel == EditLevel.Geometry)
				return;

			Object[] selection = Selection.GetFiltered(typeof(Transform), SelectionMode.TopLevel);

			if(selection != null && selection.Length > 0)
			{
				Object[] duplicates = new Object[selection.Length];

				for(int i = 0, c = selection.Length; i < c; i++)
				{
					duplicates[i] = Object.Instantiate(selection[i]);
					Undo.RegisterCreatedObjectUndo((duplicates[i] as Transform).gameObject, "Shift Duplicate");
				}
			}
		}

		if(e.type == EventType.MouseUp || e.type == EventType.Ignore)
			m_IsDragging = false;
	}
}

I've improved a little bit on the script, as I needed the functioniality to retain the prefab links.

using UnityEditor;

using UnityEngine;


[InitializeOnLoad]
static class ShiftDuplicate
{
    private static bool _isDragging;

    static ShiftDuplicate()
    {
        SceneView.duringSceneGui += OnSceneGUI;
    }

    static void OnSceneGUI(SceneView scn)
    {
        Event e = Event.current;

        if(!_isDragging && e.modifiers == EventModifiers.Shift && e.type == EventType.MouseDrag)
        {
            _isDragging = true;
            scn.SendEvent(EditorGUIUtility.CommandEvent("Duplicate"));
        }

        if(e.type is EventType.MouseUp or EventType.Ignore)
            _isDragging = false;
    }
}