Filter and do not pick up repeated firestore data on flutter

Question

I have in the Firestore the following given.

user1 = {'horário':'manhã'}
user2 = {'horário':'manhã'}
user3 = {'horário':'tarde'}
user4 = {'horário':'manhã'}
user5 = {'horário':'tarde'}

I create buttons dynamically with the schedule.

bt1 = manhã
bt1 = manhã
bt1 = tarde
bt1 = manhã
bt1 = tarde

I would like to know how to do not catch repeated data getting.

bt1 = manhã
bt1 = tarde

Use the code available in the documentation of Firestore

class BookList extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return StreamBuilder<QuerySnapshot>(
      stream: Firestore.instance.collection('books').snapshots(),
      builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
        if (snapshot.hasError)
          return new Text('Error: ${snapshot.error}');
        switch (snapshot.connectionState) {
          case ConnectionState.waiting: return new Text('Loading...');
          default:
            return new ListView(
              children: snapshot.data.documents.map((DocumentSnapshot document) {
                return RiasedButton(
                                   child: Text('${document[horario]}')),
            );
        }
      },
    );
  }
}

Answer 1

As you will build a list, it is interesting that the records are filtered before mounting the list in question, this is an output:

class BookList extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return StreamBuilder<QuerySnapshot>(
      stream: Firestore.instance.collection('books').snapshots(),
      builder: (BuildContext context, AsyncSnapshot<QuerySnapshot> snapshot) {
        if (snapshot.hasError)
          return new Text('Error: ${snapshot.error}');

        switch (snapshot.connectionState) {
          case ConnectionState.waiting: return new Text('Loading...');
        default:
          return ListView(
            children: snapshot.data.documents.map((DocumentSnapshot document) => document["horario"]).toSet().toList().map((element) {
              return RaisedButton(child: Text(element)),
            }
          ).toList();
        }
      },
    );
  }
}

Note: Make the necessary adjustments, because I did not perform tests.