### Godot version

Master (4.2)

### System information

Windows 11 - Vulkan - Nvidia RTX 4070 - intel i5 13600KF

### Issue description

As you can see in the video when you try to add multiple scenes in to the 3D viewport (only with multiple scenes) the editor throws this error.

```
ERROR: Can't add child '@Node3D@18837' to '@SubViewport@9274', already has a parent '@SubViewport@9274'.
   at: (scene\main\node.cpp:1411)
```


Video: https://youtu.be/BVcCIigQJDw

This happens in the latest master (pulled on 2.11.2023).

### Steps to reproduce

Drag couple of the 3d mesh scenes into the 3D viewport and then select multiple scenes and drag them over and observer the error.

### Minimal reproduction project

[TestSceneDragAndDrop.zip](https://github.com/godotengine/godot/files/13253741/TestSceneDragAndDrop.zip)

switch (p_what) {
		case NOTIFICATION_MOUSE_ENTER:
		case NOTIFICATION_MOUSE_EXIT: {
			mouseover = (p_what == NOTIFICATION_MOUSE_ENTER);
			queue_redraw();
		} break;
		case NOTIFICATION_DRAW: {
			if (mouseover) {
				Ref<Texture2D> h_grabber = get_theme_icon(SNAME("grabber"), SNAME("HSplitContainer"));
				Ref<Texture2D> v_grabber = get_theme_icon(SNAME("grabber"), SNAME("VSplitContainer"));
				Ref<Texture2D> hdiag_grabber = get_editor_theme_icon(SNAME("GuiViewportHdiagsplitter"));
				Ref<Texture2D> vdiag_grabber = get_editor_theme_icon(SNAME("GuiViewportVdiagsplitter"));
				Ref<Texture2D> vh_grabber = get_editor_theme_icon(SNAME("GuiViewportVhsplitter"));
				Vector2 size = get_size();
				int h_sep = get_theme_constant(SNAME("separation"), SNAME("HSplitContainer"));
				int v_sep = get_theme_constant(SNAME("separation"), SNAME("VSplitContainer"));
				int mid_w = size.width * ratio_h;
				int mid_h = size.height * ratio_v;
				int size_left = mid_w - h_sep / 2;
				int size_bottom = size.height - mid_h - v_sep / 2;
				switch (view) {
					case VIEW_USE_1_VIEWPORT: {