class MainState extends State<Main> {
…

final Map<int, dynamic> map = {};

body: FirestoreAnimatedList(
        query: firestore.collection('messages').snapshots(),
        itemBuilder: (
          BuildContext context,
          DocumentSnapshot snapshot,
          Animation<double> animation,
          int index,
        ) {

    map[index] = snapshot;

          return FadeTransition(
            opacity: animation,
            child: MessageItem(
              index: index,
              document: snapshot,
              map: map,
            ),
          );
        },


class MessageItem extends StatelessWidget {

…

@override
  Widget build(BuildContext context) {
      return Container(
        child: new Row(
          children: <Widget>[
            new Container(
              child:

    _sameUser() ?
 new Icon(
      Icons.account_circle
    )
        : Container()

bool _sameUser () {
  assert(index >= 0);
  assert(map != null);
  return map[index + 1] != null && map[index + 1]['fromUser'] == map[index]['fromUser'];
}