In ShenandoahHeap::unload_classes_and_cleanup_tables(), Shenandoah cleans up various weakly held data structures during gc.

The following code:

  {
    ShenandoahGCPhase phase(full_gc ?
                            ShenandoahPhaseTimings::full_gc_purge_par :
                            ShenandoahPhaseTimings::purge_par);
    uint active = _workers->active_workers();
    ParallelCleaningTask unlink_task(is_alive, active, purged_class, true);
    _workers->run_task(&unlink_task);
  }

  if (ShenandoahStringDedup::is_enabled()) {
    ShenandoahGCPhase phase(full_gc ?
                            ShenandoahPhaseTimings::full_gc_purge_string_dedup :
                            ShenandoahPhaseTimings::purge_string_dedup);
    ShenandoahStringDedup::parallel_cleanup();
  }

does string deduplication twice, once during the ParallelCleaningTask and after that directly again.