### 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)

switch (_edit.plane) {
		case TRANSFORM_VIEW: {
			switch (_edit.mode) {
				case TRANSFORM_TRANSLATE:
					motion = Vector3(1, 0, 0);
					break;
				case TRANSFORM_ROTATE:
					motion = spatial_editor->get_gizmo_transform().basis.xform_inv(_get_camera_normal()).normalized();
					break;
				case TRANSFORM_SCALE:
					motion = Vector3(1, 1, 1);
					break;
				case TRANSFORM_NONE:
					ERR_FAIL_MSG("_edit.mode cannot be TRANSFORM_NONE in update_transform_numeric.");
			}
			break;
		}
		case TRANSFORM_X_AXIS:
			motion = Vector3(1, 0, 0);
			break;
		case TRANSFORM_Y_AXIS:
			motion = Vector3(0, 1, 0);
			break;
		case TRANSFORM_Z_AXIS:
			motion = Vector3(0, 0, 1);
			break;
		case TRANSFORM_XY:
			motion = Vector3(1, 1, 0);
			break;
		case TRANSFORM_XZ: