### 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)

xform_dialog = memnew(ConfirmationDialog);
	xform_dialog->set_title(TTR("Transform Change"));
	add_child(xform_dialog);
	VBoxContainer *xform_vbc = memnew(VBoxContainer);
	xform_dialog->add_child(xform_vbc);
	Label *l = memnew(Label);
	l->set_text(TTR("Translate:"));
	xform_vbc->add_child(l);
	HBoxContainer *xform_hbc = memnew(HBoxContainer);
	xform_vbc->add_child(xform_hbc);
	for (int i = 0; i < 3; i++) {
		xform_translate[i] = memnew(LineEdit);
		xform_translate[i]->set_h_size_flags(SIZE_EXPAND_FILL);
		xform_translate[i]->set_select_all_on_focus(true);
		xform_hbc->add_child(xform_translate[i]);
	}
	l = memnew(Label);
	l->set_text(TTR("Rotate (deg.):"));
	xform_vbc->add_child(l);
	xform_hbc = memnew(HBoxContainer);
	xform_vbc->add_child(xform_hbc);
	for (int i = 0; i < 3; i++) {
		xform_rotate[i] = memnew(LineEdit);
		xform_rotate[i]->set_h_size_flags(SIZE_EXPAND_FILL);
		xform_rotate[i]->set_select_all_on_focus(true);
		xform_hbc->add_child(xform_rotate[i]);
	}
	l = memnew(Label);
	l->set_text(TTR("Scale (ratio):"));
	xform_vbc->add_child(l);