### 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)

/
					} break;
					case VIEW_USE_2_VIEWPORTS: {
						draw_texture(v_grabber, Vector2((size.width - v_grabber->get_width()) / 2, mid_h - v_grabber->get_height() / 2));
						set_default_cursor_shape(CURSOR_VSPLIT);
					} break;
					case VIEW_USE_2_VIEWPORTS_ALT: {
						draw_texture(h_grabber, Vector2(mid_w - h_grabber->get_width() / 2, (size.height - h_grabber->get_height()) / 2));
						set_default_cursor_shape(CURSOR_HSPLIT);
					} break;
					case VIEW_USE_3_VIEWPORTS: {
						if ((hovering_v && hovering_h && !dragging_v && !dragging_h) || (dragging_v && dragging_h)) {
							draw_texture(hdiag_grabber, Vector2(mid_w - hdiag_grabber->get_width() / 2, mid_h - v_grabber->get_height() / 4));
							set_default_cursor_shape(CURSOR_DRAG);
						} else if ((hovering_v && !dragging_h) || dragging_v) {
							draw_texture(v_grabber, Vector2((size.width - v_grabber->get_width()) / 2, mid_h - v_grabber->get_height() / 2));
							set_default_cursor_shape(CURSOR_VSPLIT);
						} else if (hovering_h || dragging_h) {
							draw_texture(h_grabber, Vector2(mid_w - h_grabber->get_width() / 2, mid_h + v_grabber->get_height() / 2 + (size_bottom - h_grabber->get_height()) / 2));
							set_default_cursor_shape(CURSOR_HSPLIT);
						}
					} break;
					case VIEW_USE_3_VIEWPORTS_ALT: {
						if ((hovering_v && hovering_h && !dragging_v && !dragging_h) || (dragging_v && dragging_h)) {
							draw_texture(vdiag_grabber, Vector2(mid_w - vdiag_grabber->get_width() + v_grabber->get_height() / 4, mid_h - vdiag_grabber->get_height() / 2));
							set_default_cursor_shape(CURSOR_DRAG);
						} else if ((hovering_v && !dragging_h) || dragging_v) {
							draw_texture(v_grabber, Vector2((size_left - v_grabber->get_width()) / 2, mid_h - v_grabber->get_height() / 2));
							set_default_cursor_shape(CURSOR_VSPLIT);
						} else if (hovering_h || dragging_h) {