### 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)

preview_camera->connect("toggled", callable_mp(this, &Node3DEditorViewport::_toggle_camera_preview));
	previewing = nullptr;
	gizmo_scale = 1.0;
	preview_node = nullptr;
	bottom_center_vbox = memnew(VBoxContainer);
	bottom_center_vbox->set_anchors_preset(LayoutPreset::PRESET_CENTER);
	bottom_center_vbox->set_anchor_and_offset(SIDE_TOP, ANCHOR_END, -20 * EDSCALE);
	bottom_center_vbox->set_anchor_and_offset(SIDE_BOTTOM, ANCHOR_END, -10 * EDSCALE);
	bottom_center_vbox->set_h_grow_direction(GROW_DIRECTION_BOTH);
	bottom_center_vbox->set_v_grow_direction(GROW_DIRECTION_BEGIN);
	surface->add_child(bottom_center_vbox);
	info_label = memnew(Label);
	info_label->set_anchor_and_offset(SIDE_LEFT, ANCHOR_END, -90 * EDSCALE);
	info_label->set_anchor_and_offset(SIDE_TOP, ANCHOR_END, -90 * EDSCALE);
	info_label->set_anchor_and_offset(SIDE_RIGHT, ANCHOR_END, -10 * EDSCALE);
	info_label->set_anchor_and_offset(SIDE_BOTTOM, ANCHOR_END, -10 * EDSCALE);
	info_label->set_h_grow_direction(GROW_DIRECTION_BEGIN);
	info_label->set_v_grow_direction(GROW_DIRECTION_BEGIN);
	surface->add_child(info_label);
	info_label->hide();
	cinema_label = memnew(Label);
	cinema_label->set_anchor_and_offset(SIDE_TOP, ANCHOR_BEGIN, 10 * EDSCALE);
	cinema_label->set_h_grow_direction(GROW_DIRECTION_END);
	cinema_label->set_horizontal_alignment(HORIZONTAL_ALIGNMENT_CENTER);
	surface->add_child(cinema_label);
	cinema_label->set_text(TTR("Cinematic Preview"));
	cinema_label->hide();
	previewing_cinema = false;
	locked_label = memnew(Label);
	locked_label->set_horizontal_alignment(HORIZONTAL_ALIGNMENT_CENTER);