### 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)

dragging_h = mb->get_position().x > (mid_w - h_sep / 2) && mb->get_position().x < (mid_w + h_sep / 2);
			dragging_v = mb->get_position().y > (mid_h - v_sep / 2) && mb->get_position().y < (mid_h + v_sep / 2);
			drag_begin_pos = mb->get_position();
			drag_begin_ratio.x = ratio_h;
			drag_begin_ratio.y = ratio_v;
			switch (view) {
				case VIEW_USE_1_VIEWPORT: {
					dragging_h = false;
					dragging_v = false;
				} break;
				case VIEW_USE_2_VIEWPORTS: {
					dragging_h = false;
				} break;
				case VIEW_USE_2_VIEWPORTS_ALT: {
					dragging_v = false;
				} break;
				case VIEW_USE_3_VIEWPORTS:
				case VIEW_USE_3_VIEWPORTS_ALT:
				case VIEW_USE_4_VIEWPORTS: {
					/
				} break;
			}
		} else {
			dragging_h = false;
			dragging_v = false;
		}
	}
	Ref<InputEventMouseMotion> mm = p_event;
	if (mm.is_valid()) {
		if (view == VIEW_USE_3_VIEWPORTS || view == VIEW_USE_3_VIEWPORTS_ALT || view == VIEW_USE_4_VIEWPORTS) {