Occurs in Fiber with multiple instances of the React package (which can't share current owner).
child: mixed,
    knownKeys: Set<string> | null,
  ): Set<string> | null {
    if (__DEV__) {
      if (typeof child !== 'object' || child === null) {
        return knownKeys;
      }
      switch (child.$$typeof) {
        case REACT_ELEMENT_TYPE:
        case REACT_COROUTINE_TYPE:
        case REACT_PORTAL_TYPE:
          warnForMissingKey(child);
          const key = child.key;