https://stackoverflow.com/questions/74566378/sum-listmap-to-get-distinct-list/74567917#74567917

merge-array-into-map.dart

void main() {
  List tagId = ['a', 'b'];
  List<Map<String, dynamic>> oldList = [
    {'id': 'a', 'value': 1},
    {'id': 'a', 'value': 1},
    {'id': 'a', 'value': 1},
    {'id': 'a', 'value': 1},
    {'id': 'b', 'value': 1},
    {'id': 'b', 'value': 1}
  ];

  final merged = ([{}] + oldList).reduce(
    (map, e) => map..[e['id']] = (map[e['id']] ?? 0) + (e['value'] ?? 0),
  );

  var mergedAsList =
      merged.keys.map((key) => {'id': key, 'value': merged[key]});

  print(mergedAsList);
}