added compare function

lib/model/task.dart

@@ -98,4 +98,14 @@ class Task {
       'url': url,
     };
   }
+
+  @override
+  bool operator ==(Object other) {
+    if (other is! Task) return false;
+
+    return hashCode == other.hashCode;
+  }
+
+  @override
+  int get hashCode => id.hashCode;
 }