### 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)

if (previewing) {
		return; 
	}
	EditorPlugin::AfterGUIInput after = EditorPlugin::AFTER_GUI_INPUT_PASS;
	{
		EditorNode *en = EditorNode::get_singleton();
		EditorPluginList *force_input_forwarding_list = en->get_editor_plugins_force_input_forwarding();
		if (!force_input_forwarding_list->is_empty()) {
			EditorPlugin::AfterGUIInput discard = force_input_forwarding_list->forward_3d_gui_input(camera, p_event, true);
			if (discard == EditorPlugin::AFTER_GUI_INPUT_STOP) {
				return;
			}
			if (discard == EditorPlugin::AFTER_GUI_INPUT_CUSTOM) {
				after = EditorPlugin::AFTER_GUI_INPUT_CUSTOM;
			}
		}
	}
	{
		EditorNode *en = EditorNode::get_singleton();
		EditorPluginList *over_plugin_list = en->get_editor_plugins_over();
		if (!over_plugin_list->is_empty()) {
			EditorPlugin::AfterGUIInput discard = over_plugin_list->forward_3d_gui_input(camera, p_event, false);
			if (discard == EditorPlugin::AFTER_GUI_INPUT_STOP) {
				return;
			}
			if (discard == EditorPlugin::AFTER_GUI_INPUT_CUSTOM) {