### 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)

} else if (vc == 3) {
			_menu_item_pressed(MENU_VIEW_USE_3_VIEWPORTS);
		} else if (vc == 4) {
			_menu_item_pressed(MENU_VIEW_USE_4_VIEWPORTS);
		} else if (vc == 5) {
			_menu_item_pressed(MENU_VIEW_USE_2_VIEWPORTS_ALT);
		} else if (vc == 6) {
			_menu_item_pressed(MENU_VIEW_USE_3_VIEWPORTS_ALT);
		}
	}
	if (d.has("viewports")) {
		Array vp = d["viewports"];
		uint32_t vp_size = static_cast<uint32_t>(vp.size());
		if (vp_size > VIEWPORTS_COUNT) {
			WARN_PRINT("Ignoring superfluous viewport settings from spatial editor state.");
			vp_size = VIEWPORTS_COUNT;
		}
		for (uint32_t i = 0; i < vp_size; i++) {
			viewports[i]->set_state(vp[i]);
		}
	}
	if (d.has("zfar")) {
		settings_zfar->set_value(double(d["zfar"]));
	}
	if (d.has("znear")) {
		settings_znear->set_value(double(d["znear"]));
	}
	if (d.has("fov")) {
		settings_fov->set_value(double(d["fov"]));
	}