fix(afs): Gracefully handle duplicate emissions on modified/deleted (angular#1825)

Author: jamesdaniels

src/firestore/collection/changes.ts

@@ -61,17 +61,21 @@ export function combineChange<T>(combined: DocumentChange<T>[], change: Document
       }
       break;
     case 'modified':
-      // When an item changes position we first remove it
-      // and then add it's new position
-      if(change.oldIndex !== change.newIndex) {
-        combined.splice(change.oldIndex, 1);
-        combined.splice(change.newIndex, 0, change);
-      } else {
-        combined.splice(change.newIndex, 1, change);
+      if (combined[change.oldIndex] == null || combined[change.oldIndex].doc.id == change.doc.id) {
+        // When an item changes position we first remove it
+        // and then add it's new position
+        if(change.oldIndex !== change.newIndex) {
+          combined.splice(change.oldIndex, 1);
+          combined.splice(change.newIndex, 0, change);
+        } else {
+          combined.splice(change.newIndex, 1, change);
+        }
       }
       break;
     case 'removed':
-      combined.splice(change.oldIndex, 1);
+      if (combined[change.oldIndex] && combined[change.oldIndex].doc.id == change.doc.id) {
+        combined.splice(change.oldIndex, 1);
+      }
       break;
   }
   return combined;