### 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)

/
	const Vector2 surface_size = p_surface.get_size();
	const real_t h = surface_size.y / 2.0;
	const real_t y = (surface_size.y - h) / 2.0;
	const Rect2 r(10 * EDSCALE, y, 6 * EDSCALE, h);
	const real_t sy = r.size.y * p_fill;
	/
	/
	p_surface.draw_rect(r, p_color * Color(1, 1, 1, 0.2));
	p_surface.draw_rect(Rect2(r.position.x, r.position.y + r.size.y - sy, r.size.x, sy), p_color * Color(1, 1, 1, 0.6));
	p_surface.draw_rect(r.grow(1), Color(0, 0, 0, 0.7), false, Math::round(EDSCALE));
	const Vector2 icon_size = p_icon->get_size();
	const Vector2 icon_pos = Vector2(r.position.x - (icon_size.x - r.size.x) / 2, r.position.y + r.size.y + 2 * EDSCALE);
	p_surface.draw_texture(p_icon, icon_pos, p_color);
	/
	p_surface.draw_string_outline(p_font, Vector2(icon_pos.x, icon_pos.y + icon_size.y + 16 * EDSCALE), p_text, HORIZONTAL_ALIGNMENT_LEFT, -1.f, p_font_size, Math::round(2 * EDSCALE), Color(0, 0, 0));
	p_surface.draw_string(p_font, Vector2(icon_pos.x, icon_pos.y + icon_size.y + 16 * EDSCALE), p_text, HORIZONTAL_ALIGNMENT_LEFT, -1.f, p_font_size, p_color);
}
void Node3DEditorViewport::_draw() {