Replicate load-as-placeholder state on node duplication

(cherry picked from commit 6aecf91e4f3d6d8a89b93fc549132a049478e73a)

scene/main/node.cpp

@@ -1963,6 +1963,7 @@ Node *Node::_duplicate(int p_flags, Map<const Node *, Node *> *r_duplimap) const
 #endif
 		node = res->instance(ges);
 		ERR_FAIL_COND_V(!node, nullptr);
+		node->set_scene_instance_load_placeholder(get_scene_instance_load_placeholder());
 
 		instanced = true;