I just figured out that in Flutter
List<int> values = [1,2,3,1,2,3];
Firestore.instance.path.updateData({"values": FieldValue.arrayUnion(values)});
results in the Firestore Array
(Firestore Array) [1,2,3]
Obviously the values are somehow mapped, removing duplicate values, although I can manually add duplicate values online in the Firebase panel (the datatype is called array respectively).
Is there a way to bypass this behaviour or is this a bug?