Occurs in Fiber with multiple instances of the React package (which can't share current owner).
returnFiber: Fiber,
    currentFirstChild: Fiber | null,
    newChildren: Array<*>,
    priority: PriorityLevel,
  ): Fiber | null {
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    if (__DEV__) {
      // 
      let knownKeys = null;
      for (let i = 0; i < newChildren.length; i++) {
        const child = newChildren[i];
        knownKeys = warnOnInvalidKey(child, knownKeys);