docs: private static final String[] deduplicateKeys(String[] keys) {
          synchronized (KEY_CACHE) {
              KEY_HULL.setKeys(keys);
              KeyContainer retrieved = KEY_CACHE.get(KEY_HULL);
              if (retrieved==null) {
                  retrieved = new KeyContainer(keys);
                  KEY_CACHE.put(retrieved, retrieved);
              }
              return retrieved.getKeys();
          }
      }