ScopedModel with Firestore adding duplicate records

I have ScopedModel which integrated with FireStore in flutter. Whenever i add or update record in FireStore data getting refreshed and adding duplicated values in model each time. How to remove existing value before FireStore reloading same.

ScopedModel - for loading intial values

Firestore.instance
    .collection('categoryMaster')
    //.where('isActive', isEqualTo: true)
    .orderBy('categoryCode')
    .snapshots()
    .listen((data) {
  data.documents.forEach((doc) {
    finalData.add(Category.map(doc));
  });
}).asFuture();

Trying to update value in sub classes, After update again same values added in model. Ex.. if i have 5 record as [a,b,c,d,e] after update 5 more values added in same list [a,b,c,d,e,a,b,c,d,e]

Firestore.instance.collection('categoryMaster')
.document(category[index].field1)
.updateData({
  "isActive": category[index].isActive
      ? false
      : true
}).whenComplete(() {
  setState(() {
    print(category.length);
  });
}).catchError((e) {
  print(e);
});

how to avoid this duplicate values form model?