fixed odd orders

src/tree.py

@@ -303,6 +303,9 @@ class TreeWalker:
         for e in config.get("export", []):
             full_name = os.path.join(section_path, e)
             exercise = load_json(full_name)
-            if "exercise_id" not in exercise:
-                exercise["exercise_id"] = uuid.uuid4().hex
+            if "exercise_id" not in exercise or exercise.get("exercise_id") in id_set:
+                eid = uuid.uuid4().hex
+                exercise["exercise_id"] = eid
                 dump_json(full_name, exercise, True, True)
+            else:
+                id_set.add(exercise["exercise_id"])