### 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)

bool has_visible = false;
	bool first_visible = false;
	for (int i = 0; i < context_toolbar_hbox->get_child_count(); i++) {
		Control *child = Object::cast_to<Control>(context_toolbar_hbox->get_child(i));
		if (!child || !context_toolbar_separators.has(child)) {
			continue;
		}
		if (child->is_visible()) {
			first_visible = !has_visible;
			has_visible = true;
		}
		VSeparator *sep = context_toolbar_separators[child];
		sep->set_visible(!first_visible && child->is_visible());
	}
	context_toolbar_panel->set_visible(has_visible);
}
void Node3DEditor::set_can_preview(Camera3D *p_preview) {
	for (int i = 0; i < 4; i++) {
		viewports[i]->set_can_preview(p_preview);
	}
}
VSplitContainer *Node3DEditor::get_shader_split() {
	return shader_split;
}
void Node3DEditor::add_control_to_left_panel(Control *p_control) {
	left_panel_split->add_child(p_control);
	left_panel_split->move_child(p_control, 0);
}
void Node3DEditor::add_control_to_right_panel(Control *p_control) {