Fix cloning entities with multiple linked_spawn relationships

[eugineerd]

Objective

#17687 changed recursive cloning to queue cloning commands for entities in linked_spawn RelationshipTargets. However, if there are multiple relationships with linked_spawn on the same entity, multiple clone commands will be queued for the same entity.

Solution

Maintain a list of already cloned entities during one clone_entity_mapped call and skip as necessary.

Testing

• Added a test when cloning entities with multiple linked_spawn relations.