### 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)

surftool->begin(Mesh::PRIMITIVE_TRIANGLES);
				/
				const int arrow_points = 6;
				Vector3 arrow[6] = {
					nivec * 0.0 + ivec * 0.0,
					nivec * 0.01 + ivec * 0.0,
					nivec * 0.01 + ivec * 1.0 * GIZMO_SCALE_OFFSET,
					nivec * 0.07 + ivec * 1.0 * GIZMO_SCALE_OFFSET,
					nivec * 0.07 + ivec * 1.11 * GIZMO_SCALE_OFFSET,
					nivec * 0.0 + ivec * 1.11 * GIZMO_SCALE_OFFSET,
				};
				int arrow_sides = 4;
				const real_t arrow_sides_step = Math_TAU / arrow_sides;
				for (int k = 0; k < 4; k++) {
					Basis ma(ivec, k * arrow_sides_step);
					Basis mb(ivec, (k + 1) * arrow_sides_step);
					for (int j = 0; j < arrow_points - 1; j++) {
						Vector3 points[4] = {
							ma.xform(arrow[j]),
							mb.xform(arrow[j]),
							mb.xform(arrow[j + 1]),
							ma.xform(arrow[j + 1]),
						};
						surftool->add_vertex(points[0]);
						surftool->add_vertex(points[1]);
						surftool->add_vertex(points[2]);
						surftool->add_vertex(points[0]);
						surftool->add_vertex(points[2]);
						surftool->add_vertex(points[3]);
					}