`Scene::write_to_world_with` multiple times with previously existing entities leads to inconsistent relationships.

[andriyDev]

Bevy version

f353cc3

What you did

let mut app = App::empty();
app.init_resource::<AppTypeRegistry>();
app.register_type::<Children>();
app.register_type::<ChildOf>();

let mut scene = Scene {
    world: World::new(),
};
let a = scene.world.spawn_empty().id();
let b = scene.world.spawn(ChildOf { parent: a }).id();
let c = scene.world.spawn(ChildOf { parent: b }).id();

let type_registry = app.world().resource::<AppTypeRegistry>().clone();

let mut entity_map = Default::default();
scene
    .write_to_world_with(app.world_mut(), &mut entity_map, &type_registry)
    .unwrap();

let b = entity_map[&b];
let c = entity_map[&c];

let d = app.world_mut().spawn(ChildOf { parent: b }).id();

scene
    .write_to_world_with(app.world_mut(), &mut entity_map, &type_registry)
    .unwrap();

assert_eq!(app.world().entity(d).get::<ChildOf>().unwrap().parent, b);

assert_eq!(
    &app.world()
        .entity(b)
        .get::<Children>()
        .unwrap()
        .iter()
        .cloned()
        .collect::<Vec<Entity>>(),
    &[c, d]
);

What went wrong

The relationships are inconsistent! We have a ChildOf component on d, but the Children of b does not contain d!

Additional information

This is a result of #17858.