Currently, when a DBClientCursor receives a response, it runs commandDataReceived. And as part of commandDataReceived, it calls parseCommandReplyMessage inside which _metadataReader is called once. After returning from parseCommandReplyMessage, DBClientCursor calls the metadataReader again. Therefore, the metadataReader ends up being called twice for each batch. It is safe to remove the call to metadataReader in commandDataReceived so that the metadataReader is only called once per batch.