### 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 (i % primary_grid_steps == 0) {
				line_color = primary_grid_color.lerp(secondary_grid_color, division_level_decimals);
			} else {
				line_color = secondary_grid_color;
				line_color.a = line_color.a * (1 - division_level_decimals);
			}
			real_t position_a = center_a + i * small_step_size;
			real_t position_b = center_b + i * small_step_size;
			/
			if (!(origin_enabled && Math::is_zero_approx(position_a))) {
				Vector3 line_bgn;
				Vector3 line_end;
				line_bgn[a] = position_a;
				line_end[a] = position_a;
				line_bgn[b] = bgn_b;
				line_end[b] = end_b;
				ref_grid.set(idx, line_bgn);
				ref_grid.set(idx + 1, line_end);
				ref_grid_colors.set(idx, line_color);
				ref_grid_colors.set(idx + 1, line_color);
				ref_grid_normals.set(idx, normal);
				ref_grid_normals.set(idx + 1, normal);
				idx += 2;
			}
			if (!(origin_enabled && Math::is_zero_approx(position_b))) {
				Vector3 line_bgn;
				Vector3 line_end;
				line_bgn[b] = position_b;
				line_end[b] = position_b;