update batch

http-api/app/rules/enrollment.py

@@ -125,6 +125,11 @@ def enroll(
             ttl_expiration = ttl(
                 start_dt=now_ + timedelta(days=course.access_period - offset_days)
             )
+
+            class DeduplicationConflictError(Exception):
+                def __init__(self, *args):
+                    super().__init__('Enrollment already exists')
+
             transact.put(
                 item={
                     'id': 'lock',
@@ -134,6 +139,7 @@ def enroll(
                     'ttl': ttl_expiration,
                 },
                 cond_expr='attribute_not_exists(sk)',
+                exc_cls=DeduplicationConflictError,
             )
             transact.put(
                 item={