### 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 (!p_viewport) {
		return;
	}
	Node3DEditorViewport *current_viewport = Object::cast_to<Node3DEditorViewport>(p_viewport);
	if (!current_viewport) {
		return;
	}
	int index = -1;
	bool maximized = false;
	for (int i = 0; i < 4; i++) {
		if (viewports[i] == current_viewport) {
			index = i;
			if (current_viewport->get_global_rect() == viewport_base->get_global_rect()) {
				maximized = true;
			}
			break;
		}
	}
	if (index == -1) {