title: Android paging 3 library removing .cahcedIn(viewModelScope) will throw exception
body: <p>I am working with paging 3.0.0-alpha11 library in my app.<br>
  There is a fragment that has a view model. in the view model I implemented paging data like this. there is one mutable live data for a parameter named promoId. promoId is a string that represents id of a model. i use it with another parameter named page to get a page of data from network. so the problem is that the first time i navigate to te fragment every thing works fine, but after popping the fragment and navigating to it for the second time, I get an error and app crashes.<br>
  my fragment code.<br></p>
  <pre><code>class ExploreFragment : DaggerFragment() {
  
      private var _binding: FragmentExploreBinding? = null
      private val binding get() = _binding!!
  
      @Inject
      lateinit var viewModelProviderFactory: ViewModelProviderFactoryImpl
      private lateinit var exploreViewModel: ExploreViewModel
  
      
      lateinit var commentsAdapter: LiveCommentAdapter
  
      override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
          initLists()
      }
  
      override fun onActivityCreated(savedInstanceState: Bundle?) {
          super.onActivityCreated(savedInstanceState)
         
          exploreViewModel =
              ViewModelProvider(this, viewModelProviderFactory)[ExploreViewModel::class.java]
  
          observeComments()
          commentsViewModel.getPromoComments(promoId)
          }
      }
  
      private fun initLists() {
      
          commentsAdapter = LiveCommentAdapter()
          binding.rvComments.adapter = commentsAdapter
          binding.rvSearchResults.adapter = searchResultAdapter
      }
     
  
      private fun observeComments() {
          exploreViewModel.commentsLiveData.observe(viewLifecycleOwner) { comments -&gt;
              commentsAdapter.submitData(viewLifecycleOwner.lifecycle, comments)
          }
      }
  
     
   
  
      override fun onDestroyView() {
          super.onDestroyView()
          _binding = null
      }
  }
  </code></pre>
  <p>This is my view model<br>
  also note that I don't use cachedIn(viewModelScope) after commentRepository.getCommentsByPromo(promoId?:&quot;&quot;) because it caches the data and does not call load method of paging data for the second time.<br></p>
  <pre><code>class ExploreViewModel @Inject constructor(
      @Inject @JvmField var commentRepository: CommentRepository,
  ) :
      ViewModel() {
  
  
      private val promoIdLiveData: MutableLiveData&lt;String?&gt; = MutableLiveData()
  
      var commentsLiveData: LiveData&lt;PagingData&lt;Comment&gt;&gt; = promoIdLiveData.switchMap { promoId -&gt;
          commentRepository.getCommentsByPromo(promoId?:&quot;&quot;)
      }
  
      fun getComments(promoId: String) {
          promoIdLiveData.value = promoId
      }
  }
  </code></pre>
  <p>And this is my repository code.<br></p>
  <pre><code>class CommentRepository @Inject constructor(@Inject @JvmField var retrofit: Retrofit) {
  
      private val api: CommentsApi = retrofit.create(CommentsApi::class.java)
  
      fun getCommentsByPromo(promoId: String):LiveData&lt;PagingData&lt;Comment&gt;&gt; = Pager(
          config = PagingConfig(pageSize = 20, enablePlaceholders = false,prefetchDistance = 100),
          pagingSourceFactory = { PromoCommentPagingSource(promoId, api) }
      ).liveData
  
  }
  </code></pre>
  <p>and the error i get is<br></p>
  <pre><code>E/AndroidRuntime: FATAL EXCEPTION: main
      Process: com.tivasoft.project, PID: 28896
      java.lang.IllegalStateException: Attempt to collect twice from pageEventFlow, which is an illegal operation. Did you forget to call Flow&lt;PagingData&lt;*&gt;&gt;.cachedIn(coroutineScope)?
          at androidx.paging.PageFetcherSnapshot$pageEventFlow$1.invokeSuspend(PageFetcherSnapshot.kt:88)
          at androidx.paging.PageFetcherSnapshot$pageEventFlow$1.invoke(Unknown Source:10)
          at androidx.paging.CancelableChannelFlowKt$cancelableChannelFlow$1.invokeSuspend(CancelableChannelFlow.kt:35)
          at androidx.paging.CancelableChannelFlowKt$cancelableChannelFlow$1.invoke(Unknown Source:10)
          at kotlinx.coroutines.flow.ChannelFlowBuilder.collectTo$suspendImpl(Builders.kt:344)
          at kotlinx.coroutines.flow.ChannelFlowBuilder.collectTo(Unknown Source:0)
          at kotlinx.coroutines.flow.internal.ChannelFlow$collectToFun$1.invokeSuspend(ChannelFlow.kt:60)
          at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
          at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
          at kotlinx.coroutines.EventLoop.processUnconfinedEvent(EventLoop.common.kt:69)
          at kotlinx.coroutines.internal.DispatchedContinuationKt.resumeCancellableWith(DispatchedContinuation.kt:349)
          at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable(Cancellable.kt:30)
          at kotlinx.coroutines.intrinsics.CancellableKt.startCoroutineCancellable$default(Cancellable.kt:27)
          at kotlinx.coroutines.CoroutineStart.invoke(CoroutineStart.kt:109)
          at kotlinx.coroutines.AbstractCoroutine.start(AbstractCoroutine.kt:158)
          at kotlinx.coroutines.BuildersKt__Builders_commonKt.launch(Builders.common.kt:56)
          at kotlinx.coroutines.BuildersKt.launch(Unknown Source:1)
          at kotlinx.coroutines.BuildersKt__Builders_commonKt.launch$default(Builders.common.kt:49)
          at kotlinx.coroutines.BuildersKt.launch$default(Unknown Source:1)
          at androidx.paging.AsyncPagingDataDiffer.submitData(AsyncPagingDataDiffer.kt:156)
          at androidx.paging.PagingDataAdapter.submitData(PagingDataAdapter.kt:172)
          at com.tivasoft.project.ui.explore.ExploreFragment$observeComments$$inlined$observe$1.onChanged(LiveData.kt:52)
          at androidx.lifecycle.LiveData.considerNotify(LiveData.java:131)
          at androidx.lifecycle.LiveData.dispatchingValue(LiveData.java:144)
          at androidx.lifecycle.LiveData$ObserverWrapper.activeStateChanged(LiveData.java:443)
          at androidx.lifecycle.LiveData$LifecycleBoundObserver.onStateChanged(LiveData.java:395)
          at androidx.lifecycle.LifecycleRegistry$ObserverWithState.dispatchEvent(LifecycleRegistry.java:361)
          at androidx.lifecycle.LifecycleRegistry.forwardPass(LifecycleRegistry.java:300)
          at androidx.lifecycle.LifecycleRegistry.sync(LifecycleRegistry.java:339)
          at androidx.lifecycle.LifecycleRegistry.moveToState(LifecycleRegistry.java:145)
          at androidx.lifecycle.LifecycleRegistry.handleLifecycleEvent(LifecycleRegistry.java:131)
          at androidx.fragment.app.FragmentViewLifecycleOwner.handleLifecycleEvent(FragmentViewLifecycleOwner.java:51)
          at androidx.fragment.app.Fragment.performStart(Fragment.java:2737)
          at androidx.fragment.app.FragmentStateManager.start(FragmentStateManager.java:365)
          at androidx.fragment.app.FragmentManager.moveToState(FragmentManager.java:1194)
          at androidx.fragment.app.FragmentManager.addAddedFragments(FragmentManager.java:2224)
          at androidx.fragment.app.FragmentManager.executeOpsTogether(FragmentManager.java:1997)
          at androidx.fragment.app.FragmentManager.removeRedundantOperationsAndExecute(FragmentManager.java:1953)
          at androidx.fragment.app.FragmentManager.execPendingActions(FragmentManager.java:1849)
          at androidx.fragment.app.FragmentManager$4.run(FragmentManager.java:413)
          at android.os.Handler.handleCallback(Handler.java:883)
          at android.os.Handler.dispatchMessage(Handler.java:100)
          at android.os.Looper.loop(Looper.java:214)
          at android.app.ActivityThread.main(ActivityThread.java:7356)
          at java.lang.reflect.Method.invoke(Native Method)
          at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
  </code></pre>
  <p>I would be very thankful if anybody can tell me what's wrong</p>