Apparently there are still pretty common Drupal 8 theming tasks that cannot be accomplished with the great twig_tweak module. This by the way was me giving a plug to a great little module, which makes half of all your theme preprocess hooks unnecessary.
Creating a duplicate of an entity
Creating a duplicate of an entity is easily done via the entity API method Entity::createDuplicate(). This is a convenient method if the goal is to clone an entity into a new entity, as all identifiers of the previous entity get unset when using this method.