title: Firestore doesn't store two identical values while updating an array
body: <p>Using flutter, when updating an array with 2 or more consecutive identical values, firestore only adds one.</p>
  
  <p>Example :</p>
  
  <pre><code>void updateValue() {
  var now = [new DateTime.now()];
  int pain =5;
   Firestore.instance.collection('Patient').document('bC2ML7LaQ1fWGOl37ZUq').collection('Symptom').document('2H6e99Bvrz6h0HBzgnyg')
              .updateData({ 'nauseaLevel' : FieldValue.arrayUnion([pain]), 'nauseaTime':FieldValue.arrayUnion(now)});
            }
  </code></pre>
  
  <p>When executing this function 2 times, the array "nauseaLevel" in firestore will only add one "pain" value and ignore the second.</p>
  
  <p>the array nauseaTime works as expected as the values are different.</p>