Stream has already been listened to

// ****** MyHomePageBloc ******
import 'dart:async';

import 'package:bloc_again/endereco_model.dart';
import 'package:dio/dio.dart';

class MyHomePageBloc {
  final StreamController<String> _streamController = StreamController<String>();
  Sink<String> get input => _streamController.sink;
  Stream<EnderecoModel> get output => _streamController.stream
      .where((cep) => cep.length > 7)
      .asyncMap((cep) => _searchCep(cep));

  String url(String cep) => "http://viacep.com.br/ws/$cep/json";

  Future<EnderecoModel> _searchCep(String cep) async {
    Response response = await Dio().get(url(cep));
    return EnderecoModel.fromJson(response.data);
  }
}


// ****** MyHomePage ******
import 'package:bloc_again/endereco_model.dart';
import 'package:bloc_again/myhomepage_bloc.dart';
import 'package:flutter/material.dart';

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  MyHomePageBloc bloc = MyHomePageBloc();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
        centerTitle: true,
      ),
      body: Padding(
        padding: const EdgeInsets.all(8),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.start,
          children: <Widget>[
            TextField(
              onChanged: (value) {
                bloc.input.add(value);
              },
              decoration: InputDecoration(
                hintText: 'Digite seu CEP',
                border: OutlineInputBorder(),
              ),
            ),
            Center(
              child: StreamBuilder<EnderecoModel>(
                stream: bloc.output,
                initialData: EnderecoModel(bairro: "Sem bairro"),
                builder: (context, snapshot) {
                  if (!snapshot.hasError) {
                    return Text("Erro na pesquisa");
                  }

                  EnderecoModel model = snapshot.data;
                  return Text("Bairro: ${model.bairro}");
                },
              ),
            ),
          ],
        ),
      ),
    );
  }
}