### 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)

#include "scene/3d/world_environment.h"
#include "scene/gui/center_container.h"
#include "scene/gui/color_picker.h"
#include "scene/gui/flow_container.h"
#include "scene/gui/split_container.h"
#include "scene/gui/subviewport_container.h"
#include "scene/resources/packed_scene.h"
#include "scene/resources/sky_material.h"
#include "scene/resources/surface_tool.h"
constexpr real_t DISTANCE_DEFAULT = 4;
constexpr real_t GIZMO_ARROW_SIZE = 0.35;
constexpr real_t GIZMO_RING_HALF_WIDTH = 0.1;
constexpr real_t GIZMO_PLANE_SIZE = 0.2;
constexpr real_t GIZMO_PLANE_DST = 0.3;
constexpr real_t GIZMO_CIRCLE_SIZE = 1.1;
constexpr real_t GIZMO_SCALE_OFFSET = GIZMO_CIRCLE_SIZE + 0.3;
constexpr real_t GIZMO_ARROW_OFFSET = GIZMO_CIRCLE_SIZE + 0.3;
constexpr real_t ZOOM_FREELOOK_MIN = 0.01;
constexpr real_t ZOOM_FREELOOK_MULTIPLIER = 1.08;
constexpr real_t ZOOM_FREELOOK_INDICATOR_DELAY_S = 1.5;
#ifdef REAL_T_IS_DOUBLE
constexpr double ZOOM_FREELOOK_MAX = 1'000'000'000'000;
#else
constexpr float ZOOM_FREELOOK_MAX = 10'000;
#endif
constexpr real_t MIN_Z = 0.01;
constexpr real_t MAX_Z = 1000000.0;
constexpr real_t MIN_FOV = 0.01;
constexpr real_t MAX_FOV = 179;
void ViewportNavigationControl::_notification(int p_what) {