该问题是由于在Akka Stream中尝试多次材料化同一个Substream Source(EntitySource)导致的。解决方法是使用shardRegion来创建并重新使用同一实例，避免多次材料化。示例代码如下：

// 创建shard region
val shardRegion: ActorRef = ClusterSharding(system).start(
  typeName = "MyActor",
  entityProps = Props[MyActor],
  settings = ClusterShardingSettings(system),
  extractEntityId = MyActor.extractEntityId,
  extractShardId = MyActor.extractShardId
)

// 创建Substream Source
val entitySource: Source[MyEntity, NotUsed] = shardRegion
  .ask[MyEntity](replyTo => MyActor.GetMyEntity(id, replyTo))
  .collect { case Some(entity) => entity }
  .mapMaterializedValue(_ => NotUsed)

// 使用Substream Source
entitySource.runWith(Sink.foreach(entity => {
  // 处理MyEntity对象
}))