Occurs in Fiber with multiple instances of the React package (which can't share current owner).
if (typeof key !== 'string') {
            break;
          }
          if (knownKeys === null) {
            knownKeys = new Set();
            knownKeys.add(key);
            break;
          }
          if (!knownKeys.has(key)) {
            knownKeys.add(key);
            break;
          }
          warning(
            false,
            'Encountered two children with the same key, `%s`. ' +
              'Keys should be unique so that components maintain their identity ' +
              'across updates. Non-unique keys may cause children to be ' +
              'duplicated and/or omitted — the behavior is unsupported and ' +
              'could change in a future version.%s',
            key,
            getCurrentFiberStackAddendum(),
          );
          break;
        default:
          break;
      }
    }
    return knownKeys;
  }
  function reconcileChildrenArray(