### 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)

right_panel_split->add_child(shader_split);
	viewport_base = memnew(Node3DEditorViewportContainer);
	shader_split->add_child(viewport_base);
	viewport_base->set_v_size_flags(SIZE_EXPAND_FILL);
	for (uint32_t i = 0; i < VIEWPORTS_COUNT; i++) {
		viewports[i] = memnew(Node3DEditorViewport(this, i));
		viewports[i]->connect("toggle_maximize_view", callable_mp(this, &Node3DEditor::_toggle_maximize_view));
		viewports[i]->connect("clicked", callable_mp(this, &Node3DEditor::_update_camera_override_viewport));
		viewports[i]->assign_pending_data_pointers(preview_node, &preview_bounds, accept);
		viewport_base->add_child(viewports[i]);
	}
	/
	snap_dialog = memnew(ConfirmationDialog);
	snap_dialog->set_title(TTR("Snap Settings"));
	add_child(snap_dialog);
	snap_dialog->connect("confirmed", callable_mp(this, &Node3DEditor::_snap_changed));
	snap_dialog->get_cancel_button()->connect("pressed", callable_mp(this, &Node3DEditor::_snap_update));
	VBoxContainer *snap_dialog_vbc = memnew(VBoxContainer);
	snap_dialog->add_child(snap_dialog_vbc);
	snap_translate = memnew(LineEdit);
	snap_translate->set_select_all_on_focus(true);
	snap_dialog_vbc->add_margin_child(TTR("Translate Snap:"), snap_translate);
	snap_rotate = memnew(LineEdit);
	snap_rotate->set_select_all_on_focus(true);
	snap_dialog_vbc->add_margin_child(TTR("Rotate Snap (deg.):"), snap_rotate);
	snap_scale = memnew(LineEdit);
	snap_scale->set_select_all_on_focus(true);
	snap_dialog_vbc->add_margin_child(TTR("Scale Snap (%):"), snap_scale);
	/
	settings_dialog = memnew(ConfirmationDialog);